Sure, Rust Foundation fits the criteria of a 501c6. It is not itself a commercial enterprise, but is an advocacy body for the Rust language and its users.

Rebble is not that. One of the key defining features of a 501c6 is that it exists to support other businesses that are associated, like a Chamber of Commerce. If Rebble did this then this whole issue we're commenting on the thread for wouldn't be an issue.

Also well funded. They would struggle to raise as much in terms of contributions IMO if not providing tax relief status to their contributors.