They say they support sending encrypted messages to non-PM email accounts:
When you send an encrypted message to a non-ProtonMail user, they receive a link which loads the encrypted message onto their browser which they can decrypt using a decryption passphrase that you have shared with them.
The real issue there is "that you have shared with them". Sharing a passphrase with someone in a secure way -- other than by telling them in person -- is tricky business. I'm curious how ProtonMail users plan to use this feature and what their attempted solutions will be.