Since the WG would be proposing changes that are only directly binding on the core development team, I’m okay with the core developer requirement.
If the WG doesn’t appear to be soliciting contributions from prominent users of the API and factoring them in, that’s a reason to go to the steering council with a complaint.
Being directly on the WG isn’t a prerequisite to contribute. It’s just a burden of having to take responsibility for the decisions and how they impact other competing interests.