It's not like they had to invent a new algorithm, just copy

First off, more intelligent pathing can be achieved trivially without any cpu hit. Secondly, if they really had CPU issues they should have gone with more abstraction instead of trying to model every individual sim.

More importantly, look into Starcraft 2's swarm intelligence algorithms used for their pathfinding. It is not computationally unfeasible to get proper pathfinding.