The article states in Twitter's reasoning that:

The second Tweet may also serve as encouragement to those potentially considering violent acts that the Inauguration would be a “safe” target, as he will not be attending.

Plans for future armed protests have already begun proliferating on and off-Twitter, including a proposed secondary attack on the US Capitol and state capitol buildings on January 17, 2021.

It seems possible if not likely that Twitter is closely monitoring how his followers are reacting to the tweets (at a scale that none of us are really capable of), and the permanent ban results most closely from that.

It is also possible that Twitter is trying to evade liability in case there are more attacks and there was some organization on their platform.