So who exactly has Manhattan exploded to instantly give him the win against Thanos?
Some normal humans? Tanks?
You know what else can destroy normal humans to an even greater extent than Manhattan can? The Power Gem. When the Power Gem kills someone, they are atomized with nothing left over (as shown when the Collector's ***** grabbed it it also destroyed the entire building). Manhattan leaves a heap of gore around, so the destruction is obviously not as total.
Thanos can touch the Power Stone without any problem at all.
But Manhattan is going to one-shot him?
At worst this is a stalemate. At best, Thanos might be able to use the Soul Stone or Mind Stone to put the Doc down.