Would not call it a guess, it is a simple linear system to scale PL such that total kJs per workout (of the same duration) roughly remain constant before and after an FTP change.
Yes, described that in the post below and seems to makes sense. Suppose you are struggling to complete workouts at a certain FTP and PL and want to reduce intensity. If you reduce FTP manually but AT would simultaneously increase your PLs and effectively keep you at the same total kJ per workout that would be pretty frustrating and lead to many complaints.