Mmo is not always tied to classic aerodynamic Mach effects (wing shock waves, etc.)
It may also be limited by structural strength (windscreen resistance to air flow or bird strikes) or by micro-Mach problems (shock waves over rivets, that aren't an immediate significant problem in themselves, but may cause vibration, leading to accelerated metal fatigue).
Mmo(perating) is just that - an operating limit. In many cases it may be a fairly significant limit like Vne - but it can also simply be like a highway speed limit (yes, you can safely go faster - but don't do it!)
But, like you, I'd be glad to hear from MIII pilots/engineers who know of the specific reason(s) for the Metros' limit.