This letter is just a continuation of the Republicans' kangaroo court disguised as an impeachment inquiry. Republicans have hit a dead end, so they're desperately creating a spectacle by calling the president to testify about something completely fabricated. If Republicans had anything on Biden they'd vote to upgrade this inquiry into a full-scale impeachment, but they haven't.
Upgrading this from an inquiry to a full-scale impeachment would be useless because it would die in the Democratic-majority Senate. It's more important to keep accumulating evidence against Biden and refer the case to the Justice Department to hold the president accountable. If Biden did nothing wrong, he should clear up any discrepancies by testifying in Congress.