The author is trying a too hard to make a point based on their belief that Proton Mail is never unencrypted at rest. That the company never allowed such a thing. And launches an attack based on this premise.
However, it's just not true.
The Proton Mail bridge which has been around for quite some time, will leave email unencrypted at rest just by the nature that it integrates with email tools like Thunderbird and X1 Search.
So this premise, "but Proton’s email famously does not work like that. There is never plain text at rest" that is used to prove the Proton is allowing unencrypted text under certain conditions solely because they consider making money more important than privacy is a false premise. Proton never made money on the bridge, for example, but it's one case where they allowed users to decide whether it was OK to have copies of emails unencrypted at rest.
My conclusion is that the author just wants to rant against Proton.