Thanks @The About Average Jake.
We rate the players for the positions in which they have been seen to play. Guardiola has only used Silva in a back four formation (thus involving full-backs, rather than wing-backs). Of course, the idea of a "wing-back" is open to interpretation, as some see full-backs who are attack-minded, or who have been given and attacking remit, as wing-backs. In the FM positional sense of the term, wing-backs are used when the manager employs a formation that sets up with three centre backs.