Is it just because the tweets in question weren’t very popular? In Media Matters’ screenshot, the ad-adjacent tweets only seem to have a few hundred views, so perhaps it was pure chance that they didn’t appear next to those specific ads for anyone else, though that seems unlikely.
Or is it because there’s an algorithm that tries to steer ads away from appearing next to potentially-objectionable content? But then why didn't it work for Media Matters’ test account? Does it have a threshold where if there’s so much potentially-objectionable content that it can’t find any valid placements, it loosens the rules? If so, how often does that happen? Or was there some other kind of failure?
In any case, I’m not feeling the transparency here.
And of course it’s hard to know what to conclude from data that’s apparently only about three(?) individual tweets, rather than a representative sample of similar tweets.