Iron Golems are extremely slow to pursue any creature that is one block higher or lower than themselves. Creatures on the same level are correctly targeted. When they or their target are in "wandering" mode, however, the golems easily navigate one-block terrain.

I tested by loading a new world with the latest Minecraft version and placing an iron golem in a 1-block deep hole. After that, I placed a PigZombie outside of the hole and the golem stared at it but did not jump up to fight it.
I then placed an Iron Golem on the same level as the pigman and it correctly targeted the pigman.
Next I tried placing slabs for the golem in the hole, but that did not convince it to move toward the pig zombie either