Twitter ceased to become a platform when they changed the content of The President's tweet. They literally editorialized his content. That act made them a publisher.
They also cut off the corners on his profile picture, listed a bunch of replies from users in a particular order (almost definitely not curated by humans, but it _could_ be?), reserve the right to make those replies more or less visible in relation to the tweet, added some widgets which make it very easy to report (and hide) an abusive tweet... The content he provided is a <=280 character string. It is reproduced verbatim.
