How strong a ground loop would necessitate the use of isolation transformer in Ethernet?
The ground connection does not go via the transformer isolation, so it has nothing to do with the formation of the ground loop.
So assume you have two PCs, both are connected to grounded mains outlets, so they both have grounded chassis, and both have transformer isolated Ethernet interface as usual.
If the Ethernet interface connectors have a metal shell that has a connection to PC chassis, and you have a STP cable that also provides the shield connection between the metal shells, then the grounds of the two PCs are also connected via the STP cable, and there is a ground loop.