Legal tender laws are likely more qualitatively significant than ability to pay taxes in of itself.

I'd be wary of the chartalist/state theory of money, since it's somewhat of a truism (state theory of money is correct for money chartered by the state). It tends to ignore that many local currencies have also emerged through different channels.

MMT as promoted by Mosler, Wray, Kelton and Tcherneva is also a syncretic and often uncredited mix of a variety of economic ideas, generally as a clout for the policy activism that its founders promote.