The good thing about an MP7 is that the bolt is tiny and therefore weighs very little, probably even less than the total weight of the slide+internals on a TM gas pistol. That all combines to make an airsoft weapon which will be more efficient, less prone to wearing out/breaking, and highly skirmishable. So it's an ideal starting place for TM to get in to the gas SMG market really.
That and they cater to the japanese market and those guys love doing USSF (SEALs, DELTA etc) Impressions even more than people here or in the US, so a realistic functioning MP7 is a solid business move.