Well, this isn't exactly proof, and I'll tell you why.

In their signup process, they might grab the pwd you entered in the registration form, generate the email, with password. Then send that email directly without storing it, while at the same time, inserting your password with an encryption into a database. Is it a good idea to send passwords by email? Well, no, but that's a different story. I don't know the actual implementation so you might be correct. But this isn't proof. What kind of email do you get if you need to reset your password by the way?