If there are incriminating DMs in Trump's Twitter Account, here is how the government would know:

A cooperating witness turned over incriminating DMs.

Also nobody knows what the DOJ knows which is one reason investigations are done away from the public view.

So if you try to suppress something that the DOJ knows exists . . . ding.

Similarly, they'd have reason to think someone else was putting out some of his key Tweets concerning the leadup to January 6 from evidence already uncovered.