Don’t do too many things at the same time.
Apparently, montaa was only accepted as proper Finnish a few decades ago, and there’s no mention of it in my textbooks, so it doesn’t come naturally to me.
My understanding is that here it’s used because monta would be used in the non-negated context: “tee monta asiaa kerralla”, right?
I think you’re right.
monta used to be the partitive of moni, but has also developed into a quantifier pronoun of its own that works like a numeral, and turns into its partitive form montaa where numerals would too.
This also means monta cannot be used with uncountables, unlike paljon.
The official explanation is here: VISK - § 765 Irtautunut moni-pronominista
Examples from that article:
Opin kaksi uutta asiaa.
Opin monta uutta asiaa.
Opin kahta uutta kieltä.
Opin montaa uutta kieltä.