Thought bouncing around my almost empty skull:
Pelosi is power hungry and likes the attention.
The U.S. (those pulling Biden's strings) wants to show support of Tiawan, thumb their nose at China, and put on a big front for the fall election.
Someone, Pelosi or others, came up with this idea, political bantering draws attention, and as a safety net to appease China over the diplomacy Biden won't commit publicly.