Slightly worrying to see the word "checksum" used to refer to a one-way cryptographic hash function...
Huh? The only mention of a checksum also acknowledges it's possible to brute-force reverse it, aka not one-way.
A checksum is a computationally efficient way to detect unintentional changes in data. But as protection against intentional changes it's quite useless. That's what cryptographic hash functions are for.
Now the proposal described using a "checksum" to tie together two halves of a self-signature, where one half would be a an identity and the second would be an email address, essentially. That would make it trivial to forge a second half with another email address (but the same "checksum").
I understand that's an entirely different problem. All I'm saying is that few cryptographers would use the word "checksum" at all, and even fewer would use it in this context. That's what worries me slightly.