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the best corner will have zero stats, because....  no one has the guts to throw his way

Then Charles Woodson and Deion Sanders must have sucked!

Again, you have to look at the rest of the roster.

Let's say you're 2021 Purdue, and you're up against Michigan with Woodson. Woodson is going to shadow David Bell all game long.

That doesn't mean you don't throw Bell's way, because nobody else on the roster is anywhere near as good of a receiver as Bell... AND because it's not like the rest of Michigan's defense sucks.

It probably means you don't throw Bell's way as often, but you're not going to win not throwing to him at all.

Now, let's say you're 2021 Ohio State. You don't throw Woodson's way, because you have three incredible receivers and whoever isn't guarded by Woodson is probably in a positive mismatch. You have the luxury of not throwing that way.

Maybe sites like PFF are trying to adhere their NFL stuff onto college, but you cant really do that. 
NFL teams are very close in terms of quality, say 40-60 on a scale from 0-100.  College football teams are more like 1-99 on that scale.  And related to that, NFL schedules are much more even, my an order of magnitude when compared to college.

.................

% of passes faced or defended would be a useful stat to have widely available.  For a Karlaftis or Vince Wilfork, number of blockers per snap would be useful.  Number of sacks or TFL while unblocked would be interesting to know, too.

Certainly not any sillier than voting total tackles of whatever stat there is. PFF is, IMO, a little sketchy for college, mostly because I just don't believe they watch every game and grade every play. But I don't doubt they watch at least the best teams and best players.