Apparently they were claiming in their SEC filings that it was under 5%. At least that was what Musk claimed.

Assuming Musk is correct, twitter must believe they the ability to at least determine the number is less than 5%. Even if they aren't able to obtain a more accurate number, being able to demonstrate that is sufficient to disprove a claim that it's 20%.

Or they're making unsubstantiated claims to investors through their SEC filings, which would seem to be a big no-no.