I am going to skip the summarizing and assume you are going to listen to the podcast, partially in the interest of remaining (blessedly) short, and partially because the talk remains focused enough that there is really just one point to discuss.
Long story short, Seidman claims that the United States Constitution, and constitutions in general, are pointless at best, and authoritarian at worst. The first point he supports by demonstrating that legislators often completely ignore the C., and often times in ways that are good points. For example, the Louisiana Purchase was pretty certainly not constitutional, but might have been a good thing to do. Likewise, many things the Federal government does are blatantly unconstitutional but it doesn't seem to matter because the C. can't order people around or control troops etc to enforce itself. Other countries don't have constitutions and they don't explode, so how do we know that they are useful?
The second point is that saying "You can't do X because it is unconstitutional!" is rather authoritarian and anti-thought, and that you shouldn't rely on that but rather try to convince your interlocutor of your points. That's the nutshell version.
So, here is the over all point Seidman misses I think: the point of constitutions is to take certain matters off the table entirely. Which matters are key, and some constitutions are better than others in this regard, but the important point is that having every issue up for debate is dangerous.
Take for instance slavery, one of the glaring omissions of our C. Imagine if every 50 years or so we had to have a debate about whether or not to reinstate slavery. From a purely economic perspective this would be quite wasteful as you would have people employed in studying and coming up with reasons for and against slavery, presumably smart people who could be doing something more productive. From a terrifying public policy perspective, what happens if the third or fourth time around there isn't a good orator on the anti-slavery side, but a brilliant and charismatic spokesman for the pro-slavery side? Sooner or later this is going to happen, which means sooner or later slavery will come back in vogue, even if under a slightly different name.
If that example seems far fetched, consider the debates between J.M. Keynes and F.A. Hayek. Arguably Hayek won the intellectual debate, but Keynes just smashed him in the realm of public opinion, due in large part to his relatively simple message and massive personal charm. If English hadn't been Hayek's second language, or Keynes had a stutter, things might have been very different.
The roots of Seidman's misunderstanding of this point is his misunderstanding of collective vs individual action, and his seeming conflation of negative and positive rights.
He often says "we want" or what "we can do" when describing the actions of governments, but the key point he thus misses is that governments are made of individual people, and even when acting as representatives have their own incentives and goals that do not, and can not, align with every one of their constituents. While it is certainly possible that this works out fine, it is more likely that politicians will use this to their own ends. Constitutions limit their ability to do this.
Counter intuitively, however, this defection by governmental agents is not the greatest danger inherent in governments. Rather, it is when governments do exactly what their voters want in times of passion that cause the most damage. By his own admission late in the podcast he says he does "not generally take a position on unrestrained majoritarianism" (right around 47:30). However, that is exactly what constitutions are meant to do, limit the harms that the majority can inflict on minorities and indeed themselves, just because it seemed like the best idea at the time. No one wants to be in a position where they have to argue persuasively as to their right to live with a lynch mob who thinks otherwise.
That last point is the junction of his misunderstanding of state action and the nature of rights. He often speaks in terms of positive rights, using phrases such as "you have to do X" instead of negative of the form "you can not do X to me." In so far as a constitution enumerated positive rights, I would agree that it would be authoritarian. However, negative rights are most certainly not authoritarian, as they do not require actions, but rather demand that you not do certain ones. Requiring that the government not kill you without a trial is not authoritarian, it limits the authority of government. Again, is it really preferable that we have to persuade others that they don't have the legal right to kill us? Consider the recent drone strikes against US citizens without any warrant or judicial review, and how Obama has yet to be thrown out of office before you decide that you can trust your fellow citizens to uphold your right to life of their own accord.
Also, as an aside, Seidman often makes the case that while aspects of the Constitution made sense 200 plus years ago, arguing that the decision that was made then still applies is authoritarian. He does not, however define at what point in time such decisions go from the legitimate will of the people to authoritarian nonsense. Which is to say, how often should we be willing to scrap everything and start over? 240 years is apparently too long, so is it 150? 50? A few weeks?
The US C., for all its many faults, seems designed to be a permissive rule set of what the government can do, which is to say what the national majority can do, and taking everything else off the table at the national level. I think Seidman is correct that ours has some serious issues, and only perpetuates as well as it does due to the fact the people seem inclined to agree with the general idea, but this is not due to an inherent flaw in the overall concept of constitutions, but with this particular manifestation. Seidman's lack of clarity on the subjects of positive and negative rights and governmental action (in addition to what level that action occurs on, which I didn't touch upon) prevent him from seeing that there have to be objective laws that limit the actions the government or a majority can take, if only because at the time of decision we can not rely on cool headed reasoning to restrain ourselves and our agents.