But you're gullible enough to think that Nixon would only have insight into transfers by getting a call from Wilder...?
Lots of people talk. Most don't have the full picture. But they will have "insight" to an extent. Nixon will have a network of people who can provide insight into certain transfers by knowledge of clubs and/or players. Given his track record on us, I'd guess he doesn't have a great insight on the club's dealings directly. But he'll be able to piece things together from people with links (of various strength) to the player, the selling club, and others in the market who are keen and therefore hear stuff too.
So yeah, he probably won't get everything spot on. But it's mad to dismiss him by saying "stunned to think people believe Chris Wilder would tell a Sun journalist about his dealings". It's a dramatic simplification.