You are correct that this loophole exists, but it is equally easy (perhaps more so) for Democrats to take advantage of it. The District of Columbia votes uniformly Democratic, which allows you to skip the step involving coordinated legislation in dozens of state legislatures. This Harvard Law Review note from 2020 lays out the process:
1. Democrats gain control of both Houses and the Presidency
2. Democrats vote to divide the District of Columbia into 127 separate states
3. Congress votes to admit these new states into the Union
[remaining steps are identical]
Winning a trifecta in the first place is probably easier for Republicans, who win national elections slightly more often than Democrats do (and also have a large Senate advantage). Point taken, though.
