It's confusing.

From what I understand, it's "backwards-compatible" with JSON because valid JSON is also valid JSON5.

But it's not "forwards-compatible" precisely because of comments etc.

Stating `backwards compatible` at a feature is at best confusing for readers who haven't already bought into JSON5 as "the way".

If people tended to interpret English correctly and not be susceptible to misinterpreting written statements, that would be nice. Reality is a bugger!