Twitter isn't a public forum for speech. You need to have an account to use Twitter, for which you agree to abide by Twitter's terms of service, in which Twitter reserves the right to moderate and ban content as they see fit. Marsh v. Alabama applied to physical property and AFAIK hasn't been definitively extended to online "properties" like Twitter.

In Manhattan Community Access v. Halleck[0], however, the Court ruled against the premise in regards to a public access television station, and maintained that the station remained a private actor despite being a "public forum."

[0]https://en.wikipedia.org/wiki/Manhattan_Community_Access_Cor...