But that is absolutely not the point, and it is not a nitpick. The real argument is that Bitcoin has negative value if you include externalities, and that is not the case for most assets.

It's not a nitpick, he's referring to Pigovian Taxes.

https://en.wikipedia.org/wiki/Pigovian_tax

It's obvious that you can tax anything by some amount to eat up the value.

He means if you tax it according to the amount of unaccounted-by-the-market negative value, it comes out negative.

Right, that's fair to say.

But the economist in OP is essentially ignoring where bitcoin actually get's it's value from and only focusing on it's electrical consumption.

Bitcoin means different things to different people. But many see it as the single best hedge against modern monetary policy.

The cost of releasing carbon into the atmosphere is there whether or not you price it into the market by use of a tax. Value is not limited to monetary value.