You've misunderstood my claim. But since you want to go into the legal technicalities, let's go there. There actually was no majority opinion in US v. Alvarez. There was an opinion by Justice Kennedy for himself and three other justices, which talks a lot about content-based discrimination. The idea here is that lies are a subcategory of content-based discrimination. Suppose there is a statute prohibiting me from lying about how many chairs there are in this room, and I assert that there are three chairs in this room, when there are in fact only two. I have violated the statute. But had I made a different claim on the same topic, had I asserted that there are only two chairs in this room, I would not have violated the statute. That makes the statute content-based. 

The controlling opinion in US v. Alvarez is actually the opinion by Justice Breyer, not Justice Kennedy, and Justice Breyer more or less skips over the whole issue of whether it is content-based, but ends up applying strict scrutiny anyway. According to Justice Breyer's controlling opinion, regulations of false speech in areas that “would present a grave and unacceptable danger of suppressing truthful speech”, such as “philosophy, religion, history, the social sciences, the arts, and the like”, get strict scrutiny. Regulations of “false statements about easily verifiable facts that do not concern such subject matter” get intermediate scrutiny, which means they still might not be constitutional.

Both opinions recognize that there are a lot of specific categories of lies, such as perjury which you mention, that are generally thought to be proscribable, and which US v. Alvarez does not touch. Neither opinion suggests that these categories of lies are somehow content-neutral. Even for content-based regulations, courts then have to ask whether the government has a compelling interest in prohibiting the speech, and whether the prohibition is narrowly tailored to that compelling interest, before declaring a prohibition on speech unconstitutional. There are a variety of other exceptions to free speech that the Supreme Court has recognized over the years (defamation, true threats, incitement of imminent lawless action, etc.) The idea with many of the categories of presumably proscribable lies mentioned in US v Alvarez is that these categories of lies are proscribable because they generally cause significant harms, even though they are content-based. This is how Justice Breyer puts it: "I also must concede that many statutes and common-law doctrines make the utterance of certain kinds of false statements unlawful. Those prohibitions, however, tend to be narrower than the statute before us, in that they limit the scope of their application, sometimes by requiring proof of specific harm to identifiable victims; sometimes by specifying that the lies be made in contexts in which a tangible harm to others is especially likely to occur; and sometimes by limiting the prohibited lies to those that are particularly likely to produce harm." Since you mentioned perjury and lying to cops specifically, here is what Justice Breyer has to say about that: "Perjury statutes prohibit a particular set of false statements—those made under oath—while requiring a showing of materiality. See, e.g., 18 U. S. C. §1621. Statutes forbidding lying to a government official (not under oath) are typically limited to circumstances where a lie is likely to work particular and specific harm by interfering with the functioning of a government department, and those statutes also require a showing of materiality. See, e.g., §1001."

The point: US v. Alvarez actually is a serious impediment to any prohibition on lying aimed at improving the general epistemic environment of public debate, and for good reason: any such prohibition has to be enforced by the government, and allowing the government to decide what counts as a lie is a recipe for censorship. People like Donald Trump sometimes win elections, and do you want him deciding what counts as a lie and is therefor prohibited?

This argument about affidavits seems wrong to me too. I've never heard of an affidavit being used in a context where there wasn't the idea of the document being used in a court proceeding, and I'm not sure such a thing would be allowed. Can you please cite a particular statute that you think would allow a podcaster to legally bind himself with an affidavit? And if such a thing did become common, do you think courts would be willing to be the arbiters of which statements were true in podcasts, or do you think they would be unwilling to enforce what they would (rightly in my view) see as a misuse of a tool intended to protect only their own integrity? The latter seems much more likely to me.