While implementing the OAuth standard itself is not novel, many of the specific design details in this implementation are. I gave it a rather unusual API spec, an unusual storage schema, and an unusual end-to-end encryption scheme. It was totally able to understand these requests, even reasoning about the motivation behind them, and implement what I wanted. That's what convinced me.
BTW, the vast majority of JS OAuth libraries are implementing the client side of OAuth. Provider-side implementations are relatively rare, as historically it's mostly only big-name services that ever get to the point of being a OAuth providers, and they tend to build it all in-house and not release code.