I didn’t agree that a useful synonym for “inhumane,” is cruel, but I will certainly concede that to the extent you believe the terms are synonymous, you have every right to use the word. And I will also concede that it’s not simply your belief: you have a good dictionary source.
So I think the problem here is mine. In my view, “inhumane” represents several orders of magnitude beyond merely “cruel,” or “lacking compassion.” But it looks like that’s my idea, and not the dictionary’s.