I've seen folks use that rationale, but the 2A doesn't say "firearms" it says "arms" - presumably "knives, nunchucks, brass knuckles, etc" would fall under that, although that's clearly not how it's been interpreted.
The author(s) appear to take the position that knives are protected under the 2A and that many existing laws regulating knives are probably unconstitutional. At least from the bit I've had time to read so far...
Come to think of it, I don't know why that is, and I'm not arguing that it's correct.
Anyway, the real point is that there seems to be a lot more variation in laws concerning the legality of owning knives, nunchucks, brass-knuckles, etc., than there is about guns. Guns may be more highly regulated in general, but I don't know of a single state where private ownership of, say, a pistol, is illegal. But I'm pretty sure there are states where switch-blade knives are. But I'm working off memory here, so maybe I'm wrong.
But not to quibble over details... all I was originally saying is that, for whatever reason, laws on owning certain non-firearm weapons (switchblades, nunchuka, brass knuckles, saps, etc.) seem to vary wildly, whereas in general firearms ownership is legal in the entire United States.
It is an interesting situation, to be able to own, say, an AR15 or a 1911 pistol, while simultaneously not being able to own brass knuckles.