I think this is a complicated question because there are many reasons someone may have a low GPA in undergrad. My guess is that people who had low undergrad GPAs, then went on to earn high post-bac GPAs, figured out what was preventing them from achieving the high GPA in undergrad and then fixed the problem before beginning the post-bac. Maybe some people had medical issues that they addressed. Maybe some people were immature and didn't focus as much on their classes as they should have, and they have learned to prioritize studying. Maybe some people picked an undergrad institution that was a poor fit for them, and their post-bac institute is a better place for them to learn.
Personally, I'll say that my post-bac courses feel difficult because there is so much to learn, but I think most of the students in my classes are able to earn good GPAs if they put in the study time. It helps that we have awesome professors!
Hope that helps a bit 🙂