AA.com does not give any specs on the pitch or length of Flagship Business on A321T, but according to to Seatguru, the bed length is 75", with width of 19".

Compared to UA on 757, the bed length is 76", with width of 20.6". So they are basically the same seats. I think what makes the difference is the slightly wider cabin on A321, which creates a sense of spaciousness.