This stinks of hollywood accounting, like a lot of "negative carbon" plans for things that would otherwise be nonsensical for.
The premise here is that if Bitcoin mining uses an energy source that inherently captures carbon or methane, then it's "carbon negative?" This ignores the fact that the energy budget is shared. We could just as well use that same energy for something else currently on carbon-based sources. So, energy is still being wasted on crypto speculation - you've just fudged the total energy budget calculations by pretending it doesn't apply here.
Burning methane has positive effects on global warming since methane is way way waaaaaaay worse than CO2. Taking care of landfills that are largely untouched, just spewing methane is a good thing. If Bitcoin ends up having a net-negative effect on global warming, how is that a bad thing?
You're assuming that all energy sources are well-connected to a grid that can handle the electricity that is generated, that's not the case at all. You can't "use that same energy for something else currently on carbon-based sources" without major infrastructure costs.
To the sibling, while I largely agree, I think there's some argument that building out additional manufacturing capacity for renewables even if it goes towards crypto mining has beneficial knock on effects for reducing the cost of the equipment provided that the learning curve effect and economies of scale outweigh the competition for resources, which to be fair isn't guaranteed.
It's mainly the fact that Bitcoin mining sets a price floor on electricity prices. It's a buyer of last resort. Without it you'll get negative electricity prices and less investment in renewables.
