Indeed. That tweet was just the icing on the cake. I agree that Proton is an issue in that it is far more vendor-locked-in than a standards compliant mail service, but in addition to that, over the years, they have very much over-sold the degree of security their system provides (military-grade encryption anyone?). Read any honest security researcher's review of Proton and it's full of caviates about a system where they both hold your keys and provide you with a web interface. If their marketing was more reasonable, I would maybe trust them more.

If you absolutely need end-to-end encryption and have the ability to direct your correspondent to a particular service (like Proton), I wouldn't choose Email at all. If it had to be Email, you are looking at PGP or S/MIME in a client, but an e2e messenger that is hard to mess-up and has some metadata protections will be far superior in practice.