Nobody to my knowledge is sneering at you or disbelieving you.
Facebook may be able to read Yahoo cookies. That's how they found your Yahoo address--the one you didn't give them. A bit creepy, to be sure, but not necessarily nefarious. Any time you use Yahoo, and you tell Yahoo to remember your ID, a cookie is sent to your computer; if you have three Yahoo accounts, you have (at least) three Yahoo login cookies. If Facebook saw one of those, that's how they knew about the account.
