It's not; it's not even close. Every message you send, message by message, HTTP request by HTTP request, depends entirely on the security of Protonmail's servers and relies on PGP, which leaks extensive metadata and has no forward secrecy. Not only that, but because you're using SMTP email, you're always one mistake away from accidentally sending plaintext.

The good secure messaging services --- particularly Signal --- make these things impossible to screw up.

Don't use email for secrets.