“Probably those ones didn’t go to college, there’s actually a pretty sizable set that didn’t. “
Huh? I can think of two NFL players in the last 40 years who didn’t go to college (save some obscure foreign kicker). That was DT Otis Sistrunk of the Oakland Raiders (1972-8) and Arizona Cardinals DT Eric Swann (1991-2000). They were discovered playing semi-pro ball. There are a few examples of NFL players playing different sports in college, the most current is San Diego Chargers TE Antonio Gates who played basketball at Kent State.
Here lists 5 but says there are many more:
http://www.mademan.com/mm/5-nfl-players-did-not-go-college.html
I think you mostly see it in the trenches. Offensive and defensive lineman seem to be the most common to skip it going semi-pro or just getting into it through other means. With a 53 man roster, an average career of 3 years and 7 rounds to the draft there’s a lot of different ways to get into the NFL, especially for the guys that won’t be getting much glory.