If you take it as referring to the thread headline (which it wasn't), rather than to the previous post (which it was) and you might read it that way. But I agree - if I had meant to say that mouth music was the only kind of improvisation, that would have been very silly.
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Of course there's an important fourth kind of improvisation, involving the words - ranging from the one singers do, when they forget a word or a line and adjust to cover, to traditions in which whole songs can be improvised - blues for example, or dub and rap and so forth, or work songs of various sorts.

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I like it when an old thread comes back to life, at least when it's one that hadn't beaten a topic to death.

I noted a suggestion back there where Callie expressed doubt as to whether group improvisation has a role in folk music. Unless folk music is defined in a very limited way, I can't see that as being right.

Mind a lot of time when people talk about group improvisation, in jazz typically, what they actually mean is one person at a time doing a relatively improvised solo, against a prearranged background. True collective improvisation is a much rarer bird since jazz got all clevered up.