So it was aircraft N770AN - a v2 with PE that shows 29 as MCE. Seatguru reports it as having extra legroom.

It looks like a similar question was asked, but there was no reply. It does seem hard to understand how middle seats in a row would have extra legroom when the seats on the sides do not. Without a tape measure or other means to verify, I guess there's no way to know.

(AA shows the split in rows 20 and 29, seatguru does not, so there's not even consistent data.)