It’s not illegal for them to collect publicly available information, like what you post to social media. Nor is any warrant needed for that. This can only be about them wanting to acquire his private messages on the platform.
I read they also wanted to determine where tweet originated (jurisdictional reasons) and who actually sent specific tweet, as it is speculated some tweets sent from Trump’s account were not physically sent by Trump.
And of course, access to DMs were probably wanted as well.