What happens if an ISP isolates a portion of nodes? I came to believe that an ISP can easily detect full nodes, BTC connections and isolate them from reaching any other nodes.

When and if an ISP decided to do so, could they easily perform a 51% attack and do everything they want?
Yes, but this would only work if said ISP would host more than 50% of all the full nodes in the network. This is simply not the case, the nodes in the network are too widely distributed for that.