I don't think the statement is "Twitter is a public forum" but "the Twitter accounts of government officials using in their official capacity make those accounts public forums in the context of the First Amendment restrictions on governmental intervention".
So I don't think this has anything to do with what Twitter can or cannot do.
Also, can officials have private accounts?
IANAL: probably, but only as long as they don't use it in their capacity as officials. Which Trump does with @realDonaldTrump.
Additionally they only argue that @realDonaldTrump is a government account because the government itself has stated that the posts from that account represent official statements by the President. More generally, they mention cases where the FOIA applies to emails sent through a private account when the communications are discussing government matters.