Bitcoin is not a debt based asset.
Reserve Notes were once issued as an IOU for gold and silver held on account at a bank. A customer would deposit a unit of gold or silver and the bank would issue an IOU in exchange. The Reserve Note was “backed” by the gold or silver on deposit and any bearer of the note could return to the bank an exchange it for the unit of gold or silver.
Reserve Notes are debt money and require a backing to provide faith they can be exchanged for something in the future.
Bitcoin, not being a debt asset, does not require backing. You either are the owner of a bitcoin, with full rights to transferability, or you do not. There is no counter party risk.
What backs the US Dollar? This is a direct quote of the US Treasury:
Federal Reserve notes are not redeemable in gold, silver or any other commodity, and receive no backing by anything. This has been the case since 1933. The notes have no value for themselves, but for what they will buy.