To me, it doesn't make sense flooding the CGN market which as you have said is up against their own LH DUS flights, not to mention 4Us flights to CGN from MAN.
I've avoided booking WW flts from MAN as routes seem to get cancelled with little notice, so I have no confidence that flights will be operated.
Another annoyance is wanting to book MAN-BCN via CGN with 4U. The Germanwings web site automatically re-directs you to BMI Baby pages - who don't operate the route anymore.
Its pretty clear that 4U + WW haven't got their act together (yet), IMHO.