I think the argument being made is that MAN is less exposed to specific risks, such as individual airline failures (or falling out with a key customer). You are right that MAN's traffic is heavily focussed on outbound leisure (although it has a higher business component than either Gatwick or Stansted). As such it is exposed to systematic risk, eg economic downturns. But in practice, MAN's traffic is probably more robust than STN in difficult economic times, given the extreme price sensitivity of much of STN's traffic, whether inbound or outbound.