Biden's ability to enforce the law seems to be pretty constrained given the amount of time left in his term. Like asking Garland to start a prosecution isn't exactly practical. I think it's also worth noting that TikTok was complying with the ban until they were given a signal by the incoming administration that they weren't planning on enforcing it.

The text of the law isn't totally unambiguous, but I still think it's quite clear that the conditions where a 90 day extension could be granted aren't being met, so we'll have to agree to disagree on how flimsy it is.