I don’t think The Guardian is wrong here, see this passage from the article:

Comer has sent 16 letters to former Biden White House officials requesting transcribed interviews, NBC said. Metadata analysis showed that all appeared to be signed with a digitally inserted signature. Further letters requesting testimony from the White House physician Dr Kevin O’Connor and Anthony Bernal, a senior aide to former first lady Jill Biden, were also signed with digital images, NBC said.

He did not use cryptographic signatures, but images of his written signature, which I think is pretty similar to using an autopen (albeit probably much easier to detect)