This is how it is: some football guys are, and have been track-fast in the 100m. Heck, I would guess even up to 400m, but to 100m, I am certain. I have trained with some of the fastest, and they have "it". If you are clean, I don't think you can build "extra" 100m ability, I think that you can just rise to your potential--and they have as much potential as anybody. If you are dirty, it's anybody's guess.
As for big guys being seriously fast, there are football-sized world class sprinters. Fearon, HAA, Big John Teeters, Leonard Scott, etc were plenty fast. Yes I know they weren't "the fastest ever", and are not in the sub-9.8 category--but there are plenty of lighter guys in the NFL too, guys who never maxxed-out their sprinting. Remember, sprinting is something they do as an aside, and to achieve what they do is remarkable. Even though the best football requires speed and power, it is a skill game--and at many positions you are practicing cutting and faking, and reading the D and its holes, more than straight-ahead speed. That takes a lot of time, time that is taken away from sprinting.
Some of the NFL guys are no joke. This guy claiming he could be up there with the best women without any specific training is hot air, but who knows.
Here is a fun video of Clowney on 4x1 anchor. He gets blown away, but look at the size of him compared to the other guys. What did he weigh in that race, 235-240?
https://www.youtube.com/watch?v=JAO7m1IfoDQ