You're trying to apply the wrong kind of logic.
Stobarts want planning permission for a road haulage depot at CAX from the council - the only way they'll get it is by tieing it into the development of the airport.
The council want an air service to London (because it will bring 'inward investment' and 'tourists' and will 'put us on the map').
Stobarts own an airport in London and an airline.
Hence the airline owned by Stobarts sends a letter to the council saying it will fly between the two airports owned by Stobarts if the council gives planning permission.
"will this route actually make any money" is irrelevant, it's about getting planning permission.
It's that simple.