The blog post explicitly says that they think that discussion around the shading language to be "one of the most fun parts of the standardization process" (i.e.

The blog post explicitly says that they think that discussion around the shading language to be "one of the most fun parts of the standardization process" (i.e. most contentious), and they explicitly said they're deferring the issue of shading language and just accepting an existing language "for now", and that they picked the Metal Shading Language because the authors of the proposal here are using Apple platforms. I think the implication is clear that they don't expect the Metal Shading Language to actually be the final shading language after the proposal has gone through the standardization process.