In this case, I'd argue it is. Twitter is anonymous in the sense that you don't need a real name for an account, but the Twitter handle and picture is clearly presented on every Tweet and the user profile is not only very visible, it's actually pretty essential due to the follow-mechanic.
This is very different compared to more anonymous forums like 4chan, (former) YikYak and it's successor Jodel. User profiles exist in all of these, but are not presented or publicly visible and postings are not linked to the same account, excluding direct replies.
It clearly isn't because in this case the context is the OP said their rule was not to follow _anonymous accounts_. It really is just painful pedantry to do the WELL AKSHUALLY thing when the intended meaning is abundantly clear to everybody including those who have never have seen or used twitter before.
sorry to come off as pedantic: it's not my intent. everyone has different experiences on Twitter, and this relates to an experience i don't have much of.
a _lot_ of people complain about "anonymous Twitter users", and i want to understand what they mean by that. i think it's the sort of "[anonymous] asshole slides into my timeline and then leaves" behavior. and if so, i suspect it's not actually identity or its form but _reputation_ that matters in these interactions: "non-reputable asshole slides into my timeline" (and so considering reputation becomes important in your interactions). but they could equally mean "this person could have multiple identities on this site and that doesn't work for me" (e.g. some person could be playing both a left-leaning and a right-leaning account and using those multiple identities to drive a wedge into some community), so maybe they really do want to avoid interacting with people who don't have a verifiably singular identity (this isn't easy).
i should have distilled it to that point: when a person says they don't deal with anonymous users, do they actually care about identity, or are they using identity as a proxy for reputation -- and reputation is the more direct concern?
Arguing about pseudonymous vs. anonymous seems overly pedantic, and not particularly helpful.