Yes, it's exclusively because of his using his Twitter account as an official outlet for government statements.

Trump has appealed the decision to SCOTUS, which is still considering whether or not hear it [1]. Given that Trump's Twitter account is now banned, I suspect SCOTUS will dismiss the motion as mooted.

[1] See https://www.scotusblog.com/case-files/cases/trump-v-knight-f... for the docket information.