Publishing an implementation of a proposed web specification is how all web standards are created or evolve. The same thing happens with WebGPU, WASM, and many before them. Usually with a prefix (ms-, moz-, webkit-,...) and/or locked behind a config setting before standardization.

What is different this time other than it being a feature that is considered user-hostile?

That's not to say we shouldn't oppose this feature, I just wouldn't be up in arms about an implementation existing.