So the manager can be honest, but it is usually not even his call. The company will likely to put a lot of value on the maintenance side (which bring the money) and less on new projects (which get postponed if there are any issues in the maintenance side).

I do not think that the manager is lying to you. TODAY he see it as 50/50. But what happen if the cluttered project is late? What happen if there is a need for new features in the cluttered project. I would accept the job only if you are ok staying in the cluttered project team for at least a year or two.