There isn’t enough time for the current President to enforce this. A convincing pledge from the incoming guy that he’ll allow them to continue operating is all it would take. How you get a convincing pledge out of this guy, I have no idea, but apparently they believe it.

TikTok operated in a way that did not need to happen. Biden's administration was explicit in that the enforcement of the ban were to be performed by the Trump administration. Trump signaled that he would sign an EO allowing a 90 day extension to the ban terms on Monday. TikTok are now operating based on this information.

Who is currently in charge of the oval office is an irrelevant quality.

Note that the ban was not really on TikTok, but the ownership. TikTok could be owned by many other parties in the world. It just can't be ByteDance or parent/subsidiary which has ties to China.

But that is essentially what is happening. There is long-standing convention for the president elect to not step on the sitting president's toes prior to inauguration, but Trump has been bucking that convention this time around. This is just an impossible to ignore example.