But in Europe that is not sufficient to lose the right to anonymity (and it shouldn't be, it's incredibly easy to get charged, no matter whether anything bad happened).
Do leaders of racist boomer political movements deserve anonymity?
He isn't a leader of anything. Hasn't ever been (even when he was a founding member, he wasn't the leader).
He is a nobody (as he should be). And as such he deserves anonymity, yes. Just because he tried to change himself back into no-a-nobody doesn't mean he has succeeded.
Why couldn’t he keep it confined to VierChan?
Nazis are gonna Nazi.
Edit regarding your edit: yeah, that seems pretty fishy. I don't think they should have mentioned it, but with enough inside knowledge you'd probably find him by just "84 year old right-wing extremist blogger from Austria". That is (fortunately) not a huge population. I suspect (and this is purely speculation) that the authors don't think he deserves anonymity (so they include enough information to find out who it is), but do think they shouldn't "advertise" his cause (so they make it easy to ignore who he is). Similar to how media outlets in the US have finally decided to not publish the names of mass shooters: there is very little public benefit in publishing it and a very real risk of it encouraging others.