Exactly. There's almost no way these guys are slower now than they were in high school. Men mature a ton physically during that time, and the strength and conditioning is much more intense in college.
Lots of players add weight, but as you said that's mostly muscle which makes you faster, and very few players are asked to put on weight at tthe expense of speed. You're OL exception is probably correct, but I would go more extreme than that - like an Omameh situation - 235 to 305. He may have had a faster 40 in high school, or a guy who switches from TE to OT or something like that.
But most players, like a LB who puts on 30 pounds over 4 years, does not get any slower with that weight. He almost certainly runs faster, since a lot of that weight is helping him do so.