This is a Staley pick IMO. Fangio is known for being more cautious on the back end and his safeties need to be good in zone and good in two deep supporting the underneath. Fuller probably can't be a great single high type because of his athletic limitations but give him half the field and he looks pretty damn good.
Staley loaded two safeties who both showed well in what the Rams will have them doing. And of course Burgess also adds nickel and man coverage ability but it's pretty clear this was seen in-house as an area they needed to get better. IMO we'll see much better zone play this year by our DBs at large.