This is exactly the point I was trying to make, thanks.

"Thus, a ship with a turret upgrade would always measure closet point to closest point, unless firing with the primary weapon"

I think this is why they were specifically talking about turret primaries: to make it clear that if your primary weapon is a turret, you still have to treat it like a turret and don't have the option of declaring a primary weapon attack inside your firing arc to try and get the better shot in this situation.