This is context-free. Matthews is not a game breaker, he can't beat people over the top, has average hands, and isn't particularly explosive. He appears to be a solid route runner. He is not terrible. He is simply garden-variety. That's great for a #2 or #3, but not a #1, and the Bills no longer have a real #1. Watkins is ten times the talent he is, and moreover Matthews reportedly has a knee issue.
I strongly suspect that both of these guys were toss-ins, for either cap, upcoming UDFA status, or roster reasons. The real trades were for the picks.