To expand on this, if you look closely, you'll see "A321" markings next to the bottom right of the L1 and L2 doors. They're there so that gate agents won't confuse it with a 757 and to serve as a reminder not to use the L2 door.

I believe when AA was first outlining the A321T service, their initial plan was to board via the L2 door, but the idea was quickly scrapped.