Correct. I'm going to use rough example numbers because I'm too lazy to look up the actual numbers but the same principle applies with the actual numbers.
Let's say nobody cheats, for the sake of argument. Let's also assume that the BAA has 22,000 spots available for qualified entrants but they have received 28,000 applications.
All applications will be ordered by time they beat their BQ not by fastest times. So a 30 year old guy who ran a 3:04:50 will get a BQ-0:10. A 46 year old man who ran a 3:20:00 will get a BQ-05:00. A 30 year old man who ran a 2:59:59 will get a BQ-5:01.
Then they rank all 28,000 applicants by their BQ-x time and they start subtracting a second at a time. But the time they get to BQ-1:31 they eliminated 5,923 runners and they still have 22,077 runners that could meet that time standard, BQ-1:31. That would mean every 30 year old guy would have had to run a 3:03:29 or faster and every 46 year old guy had to have ran a 3:23:29 or faster.
So they cut it one more time and the new test is BQ-1:32, 3:03:28 for the open male and 3:23:28 for M45-50. With that one second drop they eliminated another 148 runners. So the field will be 21,929 runners and the time to beat is BQ-1:32. Everyone who ran BQ-1:32 or faster got in and everyone who ran a BQ-1:31 or slower was left out.
Now let's assume that there was a single cheater who "ran" faster than BQ-1:32. If you would eliminate that single person then the qualifying time would still be BQ-1:32 but the field size would drop to 21,928 and the same 148 runners with BQ-1:31 would be left out.
It wouldn't be until you eliminated 77 cheaters like Mike Rossi that you could squeeze in all 148 BQ-1:31 runners for a total field size of 22,000 if you adhere to a hard limit of 22,000.
So a single cheater in itself does not alter the cutoff but a couple dozen collectively does. Given that the cutoff is also based on registrants with self reported results and the results are not verified prior setting the cutoff, false reporting probably accounts for as much of an error as actual cheaters.
With Mike Rossi it's not that he single-handedly denied someone the chance to run Boston but that he cheated and got something that he was not entitled to while others earned their way, either by qualifying or raising money for a charity, to run Boston.