Earlier quoted context omitted.
I'm pretty sure they know what you made if you worked at a large company anyway. For small ones they can probably guess better than the job seeker can. Excluding insider info because it goes both ways.
They can guess based off ranges. They dont have a list with your name and exact salary. In fact it’s explicitly illegal for them to ask in many states so this would circumvent that law.
My line of thought is that while I can look at Glassdoor or whatever so can they. But they can also get a department to do what I'll be doing 40hrs/week. Maybe I can ask a friend at that company but there's nothing really stopping my interviewer from asking his friend at my (ex)company the same thing.