I'm not sure they'd be such a useful aquisition to the big players, as their strong point is the usability of their app itself, rather than the underlying technology so much. Citymapper already use Google Maps for walking directions and actual mapping, and Google Maps already have the same transit data that Citymapper gets from public APIs.

Citymapper takes a very different approach to UX to Google Maps so I can't see them going together well. Perhaps it could be used to improve Apple Maps, but again I think it would clash with Apple's usual way of designing apps in house.