Did you read their announcement when they banned him[0]? Banning the man representing, at minimum, a quarter of a country with such nakedly partisan logic is an embarrassment to anyone who wants to pretend Twitter cares about liberal political ideals. They were listening to voices in their head rather than reading what Trump wrote; voices which obviously won't tolerate anything outside a narrow, illiberal view.
[0] https://blog.twitter.com/en_us/topics/company/2020/suspensio...
A system that, based on their flimsy justifications, simply bans political opponents isn't liberal. The classic position a liberalist should take is that the service policy needs to be reformed to embody better values around free political speech.
Not that I'm too invested in the situation. Twitter will be on the way out soon enough if it doesn't pull its head in and tolerate serious dissent from the favoured narrative.