The AfD is a legitimate political party. Legitimate as in, they haven't been caught with anything openly anti-constitutional.
Individual members have been, and were tried in court, and if found guilty were publicly expelled from the party, hence they operate under the guise of plausible deniability for the time being.
Nazi propaganda has been blocked once it's confirmed anti-constitutional, but you can't block a political party just like that.
And blocking Twitter as a whole is quite a big difference to blocking certain individuals or groups. No matter how much crap is on there, there are still a lot of legitimate postings, not least from legit government actors etc.
No, where did I say that? All I said is that as long as they are a legitimately recognized party, they can't be censored.
I'm all in favor of banning the AfD, but only after this has been achieved, can their public channels, mouthpieces and whatever be censored, not the other way around.