But it’s not public. It’s a private blockchain. The immutable ledger aspect only matters if everyone can see the ledger. Otherwise we take at face value all of the things you said. Assume they run one node and that one node is compromised by a malicious actor. The system fails. Extend it to a limited number of nodes all controlled by SREs and assume an SRE is compromised (this kind of spearphishing is very common). The system fails again.
Sure, you can creatively figure out a way to manage the risks I’ve mentioned and others I haven’t thought of. The core issue, that it’s not public, still remains. If I’m supposed to trust Proton telling me the person I’m emailing is not the NSA pretending to be that person (as the Proton CEO suggested), I need to trust their verification system.