The point of the Northern powerhouse is that it can stand on its own two feet and decrease its perceived reliance upon the south east.
The argument that the northern powerhouse needs LHR runs contrary to that objective. Quite simply, LHR are trying the same divide and rule tactic as generations of Westminster governments - playing the regions off against each other which has the output of disproportionately benefiting the south east. The NP is trying to break this.
You're confusing two things here - from a North East perspective, any reduction in LHR's position as a global hub is more likely to benefit AMS/DXB/CDG rather than MAN. MAN doesn't have the range/frequency of services.