The fungibility argument doesn't work when you consider that not all power generation in the world is connected to the same grid. There are crypto operations that use their own renewable power infrastructure. That energy never would have made it to the grid because there was no incentive for it to be put there (otherwise it would already have been there). The incentive is to use it for crypto not your mom's blender.
Sure in theory. In reality not all energy is fungible because not all power generating systems in the world are connected. If I stand up a geo-thermal farm to run my crypto mining operation that power would not have been available to the grid anyway because the economic incentive to build the power station was crypto, not selling it to the grid.
There are plenty of crypto operations that have their own, renewable power sources that operate in a closed zero emission fashion.