I think people should first agree on what it means to be "backed by" something.

My take is that it usually refers to the promise to exchange paper IOUs for gold which gives the paper some desired scarcity.

Since bitcoin by its very nature is scarce, it doesn't need to be backed by anything, all it need to do is continue to be limited to 21 million coins. And this property of bitcoin is ingrained in its code. Proof of work and energy being required to guarantee that scarcity but by no means acting as the promise to exchange bitcoin for