I would be flabbergasted if there were more than 1% of the figures you pointed out were tankies, let alone 10%.

I said 'buy into tankie talking points'. The point isn't that the 10% (itself just a random number, of course) are tankies, but that 10% are influenced by the shite that tankies spread.

Although I would emphasize that there is a certain percentage of people who are batshit insane on any given issue, and some 10% of Americans are tolerant of fascism. 10% of Gen Z going tankie instead doesn't seem that far-fetched to me.

Also tankies don’t believe in voting

This is true, but as mentioned, it's not even about them, specifically - but what effects they have on discourse.

and are still dwarfed by other sources of misinformation.

Yeah, if I was in charge of a NGO dedicated to combating misinformation or something I wouldn't be dedicating many resources to tankies specifically.

Lets say tomorrow there are no more tankies, they magically never existed in the english speaking world, what actually changes for the US?

Support for Ukraine increases by 2%-3% points as a very loud (as you note) minority is no longer amplifying misinformation and propaganda under the guise of combating the absolutely fucked state of our society.