Perhaps the Mackem kit man was wanting the Sunderland players to feel at home.
But this sort of thing has been going on for years.
I remember one sponsor at St James' Park (the box right next to the Leazes End) - a printing company - had the box decked out in Sunderland colours because the owner was a Mackem.
When I asked him why he booked a box at St James' he told me: "We have one at Sunderland too. It is for our clients when they come on business trips, but I put up the big Sunderland team photo to wind them up."
I know Toon fans who worked on the contruction of the Stadium of Light placed all manner of Newcastle flags, hats, etc on the ground before the turf was laid.