theoretically a FR player will have a slightly tighter range OTB than someone at 6max, not b/c of the number of tables, but b/c the typical FR nitreg doesn't understand abusing position.

What you seem to be missing is that the wider someone's PF raising range the lower implied odds we have postflop. If someone is opening 25%+ of hands from the button it's going to be very hard to stack the necessary amount of their range when we do flop a set, and the other 7/8th's of the time we get to c/f.