I don't believe Elon ever stated Twitter has no right/freedom to ban (he does say a lot of bizarre things, and I don't follow too closely), but correct me if I'm wrong. Disagreeing with the Twitter policies is not the same as believing Twitter is not free to establish such policies.
Twitter is free to ban Trump or others, regardless of whether Elon Musk owns Twitter. Whether to implement a ban is a decision their management can take, or not take.