While the open source argument is valid, the end to end encryption critique is a bit odd to me. It is, by definition, end to end encrypted, and proton claims that the chat history is stored locally on your device and is blindly encrypted at rest on their servers, same as in their email system.
The individual messages do have to be decrypted for inference option arrival, but really there isn't another option.
What I'd like to see is evidence of 3rd party audits on this scheme in addition to more information on the models used.