For what it's worth, I've now seen JSONC files (with comments) stored in `.json` multiple times, e.g. the launch settings in any VS Code project. So while technically a different file type, I do wonder how you'd want to detect it based on the file ending, making the idea of two different parsers for this IMHO quite impractical.
The thing is, I want the syntax highlighting to mostly make my file readable, not to teach me how "wrong" it is when it is actually correct given the actual file type, and I would assume that applies to many users. And it seems like there is no way to safely detect here if a comment in a `.json` file is an actual mistake, or just a quirk of whatever tool wrote/reads that file. So I don't think it makes sense to attempt it, or to mark it as "mistake" by default.