I played around the rules and entered some random characters during registration process, so I managed to test register even with fake email containing only @ character with two letters, one before and one after.
This is probably not great for security, but since this is acting as a salt you can just type longer random passphrase containing one @ character, it doesn't have to be email at all.

@BlackHatCoiner and @bitmover,  I updated the signup page to be more clear about the email being used as salt.

Do you think this is better, or do you consider warpwallet with email+pw a bad idea in general?

I personally like the idea because users are very familiar with such logins.