That's not how the law works, though. Let's say Trump goes back on his word and doesn't sign this executive order, and then ByteDance (etc.) get into legal trouble. If they can convince a judge/jury that they had a strong reason to believe that they'd be acting within the law as they believe it would have been executed by the incoming Trump administration, that could be a persuasive defense.

That doesn't mean TikTok would be able to continue operating, but it could mean the parties involved wouldn't have to suffer penalties for their operation up to that point (past the ban date). But maybe it wouldn't work, and a judge/jury would throw the book at them. We just don't know until and unless it goes to court.