By this logic the uber athletic DT at a some random 3A high school that takes snaps at TE in the redzone has "positional flexibility" in college. Lining someone up at the 9-Tech spot while you have 3 other down lineman and telling them to hit mesh point as quickly as possible doesn't make some a DE. It's not like he was taking reps working on defeating hand-replacement moves, or developing a speed-to-power pass rush move. Same with LB - he's not stacking and shedding, he's not working on defeating OL blocks, he's not honing his scraping skills. The dude got lined up to the Field side of the play and got asked to hold the Boundary . . . whether that was at the Wide-9, or whatever FSU's term for a $backer is, or at either SS or FS. That FSU defense that Kelly called relied on traditional split-field safeties as well, so let's not pretend he was doing something dramatically different between FS and SS.
I personally don't think he can be a Man Corner, out wide or in the Slot, in the NFL - this is the biggest point of difference to me between him and Fitzpatrick. So if I don't think he can play Corner in man, he can't play DE, he can't play LB, he can't play the $backer if you run a lot of man ( i.e., the Cardinals), you're left w/ a guy that can play Safety ( FS or SS ). In fairness, I do think he could play that Earl Thomas Cover 1/3 FS, which not a lot of guys can do.
Positional flexibility for a DB comes down to 1 thing for me: are you the swiss army tool that allows me to change my coverage shell with just the players on the field by asking you to switch responsibilities or positions? If an inline-Y gets out of stance to become an F, can I rotate you down to man up against him? Vice-versa, if an F motions back to a Y and suddenly the offense is in 12 personnel, can I trust that you will man up against the TE and hold the boundary if there is a Sky call? Could I run a press-and-bail w/ you as my CB if the X or Z motions inside of the slot? Of these 3 things I've listed, I'd trust him to do the 2nd but that's it.
I wouldn't want a team filled w/ these guys, but having at least 1 of them on the field for you is an enormous benefit.