The IMUs that existed on aircraft
before the invention of GPS have been superseded by the ones which actually are in your phone, in much the same way and for much the same reason that a $20 Casio F-91w keeps better time than a fancy Rolex that costs more than a house in California: electronics are cheaper and better than mechanical systems.
We have, naturally, also made better IMUs for places where it matters, ones which won't fit in your phone.
The question is therefore not suited to "aircraft grade, yes or no?", it's "how expensive is the cheapest IMU that's good enough for the specific need?" which in this case itself depends on how many stops is desired.
