From what I've read is not authentication bypass, it's a RCE using certificates to deliver the payload. If a specific signature is found it runs the code that was sent in place of the signing public key. It also means that only someone who has the ability to generate that specific key signature could use the RCE.

There were some other bits that looked like they could have been placed to enable compromising other build systems in the future when they checked for xz support.