I think you're correct. My problem is that their function block doesn't let you see the individual contributions and doesn't set a bit if the proportional contribution is greater than 100%. The operators, and I, get a little confused when we see a process variable above a setpoint and the controller output steady at 100%, not coming down as it should. I'm suggesting to AB that they set a status bit so you know this is happening.
I'm still curious how other vendors handle this, though. I haven't found sufficient detail in on-line docs from other vendors to indicate how they handle clipping.