You might as well just write instructions in English in any old format, as long as it's comprehensible. Exactly as you'd do for human readers! Nothing has really changed about what constitutes good documentation. (Edit to add: my parochialism is showing there, it doesn't have to be English)
Is any of this standardization really needed? Who does it benefit, except the people who enjoy writing specs and establishing standards like this? If it really is a productivity win, it ought to be possible to run a comparison study and prove it. Even then, it might not be worthwhile in the longer run.
That said, I reckon that’s actually what this project is trying to lean into. It looks like it's just standardising where those instructions live (the SKILL.md format) so tools can find them, rather than trying to force a new schema.
Fair play to them for trying to herd the cats. I think there's an xkcd comic for this one somewhere.