E.g. you cannot yell "Fire!" in a theater and expect "Freedom of Speech" to protect you from any repurcussions.
The post doesn't appear to be disputing that the fire in a theater example is a legitimate illustration of the limits of speech under the first amendment. They just proceed from there to make a more general principle that this example can be invoked by people who intend to suppress speech. Which I also don't think is in dispute.
So I don't think that post has anything to do with anything, insofar as this comment thread is concerned.
It certainly does say that, and that's really broad and vague. And it's hard to keep track of this whole thread, but I'm pretty sure the branch we're on sprouted from a categorical claim that there are no "buts" when it comes to free speech. But your article doesn't dispute that there are exceptions, and so it's fair to invoke the fire in a crowded theater to illustrate the principle for that purpose.
For anything beyond that, the value and relevance will depend on what point is being made by whom and how well you are doing in your responsibility to interpret and respond to those charitably. But linking to a long blog post and vaguely warning that it's "bad" without doing any further work to connect that to a contention in any given comment is just confusing and unconstructive.
And people should not be idiots to be panicking from someone yelling "fire" when they can't see one in the theater...
There are definitely places in a theater, out of the view of spectators, but likely fire hazards, e.g. the projection room.
Most building fires cannot be seen until it is too late. Smoke is an early indicator of a fire, but it cannot be seen in a pitch dark room.
And even in perfect conditions to determine if there is a fire or not, the audience may not be capable to judge if there is one or not, e.g. children.
I should be immortal, all knowing and beautiful but I'll have to make do with what nature gave me.
It can very well be a principle of law. In certain legal codes, it actually is.
As for reality, that's a pretty low bar.
We have many "shoulds" (e.g. you should not cross a red light lest you get a ticket / prison) that are not some inherent property of reality or physical law, but things we've decided upon.
In fact, those are the only things that makes sense to consider as "shoulds". The constraints of reality are not negotiable, so should there is superfluous.