I don't know why you think he couldn't. A legally binding statement of intent to offer TikTok the 90-day window and work out a "deal" once in office would be more than sufficient justification for the heads of the various companies involved to ease enforcement until things become more resolved.
> A legally binding statement of intent to offer TikTok the 90-day window and work out a "deal" once in office
Would not be legally binding. The President cannot unilaterally bind the U.S., and he is free to make and break statements of intent.