I think they think this is how to cryptographically sign the binding between email address and pgp key, without needing a trusted central signing authority. That is my guess.
A "Private Blockchain" used to store pgp public keys of email addresses? Seriously wtf is wrong with you Proton??!
https://proton.me/support/key-transparency
1) Why a "private blockchain"? Why not just a database that you run on your servers?
2) Why are you reinventing pgp keyservers with absolutely no upside?
Discussion
Aren't they the central authority given that they run their own private blockchain?
One could argue that the commitment log would be auditable in theory, but in practice, who would be doing it?