BD beating Pak and Kenya beating WI were upsets. I do not think the present Zimbabwe side is as strong as the Pak and WI sides mentioned above. In the present context, Zimbabwe is not really much stronger than Kenya or USA in one dayers.
BD and Kenya had won by batting first in the matches mentioned above. USA chasing a huge target after batting second is a remarkable effort from them. I do not think a side can chase such a total by fluke. That is why I believe USA is in the same league as Zimbabwe and Kenya in limited overs cricket.