I don't think I've ever heard of a 4.75 CB

At least not a CB who ever did anything.
I am with you about the pajama olympics. That shouldn't be the determiner of much.

But he ran slow at the Combine, and then went even slower at his Pro day.
He's slow.
You can't be slow and be a CB. I don't care what the college tape says.
Jumping up in the Pros means he won't face anyone slower than about 4.6.

He simply cannot play CB in the Bigs at that speed, I don't care if he has perfect technique.