It’s not a wild conspiracy theory to say that it’s not Biden’s signature on Sunday’s letter. It’s just not. Look at the way the real Biden sign “R” in “J R Biden Jr.” Sunday’s “R” is obviously not the same.
Involuntary removal before the August convention, to bypass the Republican House of Representatives, Section #4:
https://constitutioncenter.org/blog/can-the-cabinet-remove-a-president-using-the-25th-amendment