I am also getting conflicting information from AT&T. Some reps say the throttling ("deprioritizing") occurs at 22gb per line and others say at 22gb per account. In my case, that's a huge difference.
Its per line.
Unlimited data is not shared in an account. If you have a family plan with 4 unlimited data plans you dont share any amount of data so once one line hits the 22gb mark and the others dont then only the particular line can get throttled.
The other lines will get deprioritized only after they hit that amount each.