Its not complicated. It is not necessary for crypto PoW to be powered by CO2 emitting fuels. It can be powered with renewables. Therefore the power usage of Bitcoin can't be used to infer its environmental impact. You need to know how much of that usage was on the back of fossil fuels.
Energy is fungible. Unless those renewable-generated watts were unable to be used for other things, then all marginal use of energy uses emitting sources, since we are not yet at 100% green energy. There are a few exceptions where mining is done on grids that are at 100% green energy, but this is not the norm.
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.
These cases exist, but they are not the dominant form of mining. If all mining was done in such circumstances (and the energy source couldn't be connected to a useful grid) then people wouldn't be as upset.
> It is not necessary for crypto PoW to be powered by CO2 emitting fuels. It can be powered with renewables.
That's completely irrelevant for the correlation. Renewables are theta-bound by fossil fuels, and fossil fuels are theta-bound by renewables. Only at zero emissions for a source this would stop being true. This is first semester math.
> renewable power sources that operate in a closed zero emission fashion
If you mean operational emissions, then this is a trivial statement. If you mean total emissions, then this is a wrong statement, unless these people manufactured their own generators using zero emissions in the process.
If base load were to drop due to crypto miners being turned off, the energy with highest marginal cost would be turned off, which means fossil fuels, not renewables.
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.