The PoC was made in 2016, the work started in 2017, but the first spec draft was released on 18 May 2021. [1] This first draft already contained references to WGSL. There is no reference to SPIR-V.
Why did it take this long to release the first draft? Compare it to SDL_GPU timeline, start to finish in 6 months. Well, because the yak shaving on WGSL had already begun, and was eating up all the time.
[1]: https://www.w3.org/TR/2021/WD-webgpu-20210518/