Well I found my solution - it involved spending 3 hours on EA's support chat getting my EA account to have a persona for my Xboxlive gamertag. My character didn't show up yet but my achievements showed up immediately.
To clarify, for anyone who has ever played an EA console game online, the idea is not to link the game to this site but this site to the game. Registering here either creates or retrieves an existing EA account. You were not supposed to register only to spend three hours on support; you were supposed to use your existing EA account. Enough people are having problems understanding this that I've begun to question if the registration process is not informative enough, but to me it made perfect sense.