PM’s apps perform the encryption on your own device because it’s your device that runs the apps. That is e2ee, but still only in the two scenarios I mention and even then it’s also vulnerable to targeted attack. PM could ship malcious j/s if it wanted (the likely case being to comply with a court order). It’s better if your own non-j/s FOSS MUA handles the crypto, which is actually easier if you don’t use PM.
If mailbox.org works the way anonaddy works, then that’s not e2ee. The msg payload is seen by the server that does the encryption, in the very least. The sender’s ESP would have already seen the msg.





The article doesn’t seem to answer the “why” question too comprehensively… just mentions some performance differences.
I saw a Tesla in Europe with a bumper sticker like this: “I bought this car before Elon went crazy”. So I would not neglect the cultural factor.