Yes, it is possible that in Rust you would marshal into an enum for example. In this case the point becomes moot.

However, it's not so simple in other languages. In Go for example, type Foo struct { type string } is not valid.

I think it may also be confusing to have both a "kind" and "type" on the same object. Maybe a different name can be found for the markup "kind"? Maybe "position" ("position": "open", "position": "close" etc)?

I think "kind" is also not necessarily less common than "type" in other ASTs. A difference here from for example the JS ASTs expressed in JS, is that this AST format is designed to be used in many different languages, not just JS. As such, I think it's quite important that the format is easially portable to other languages. Part of making it feel "native" to more than just JS, is that you don't have to use weird incantations like r#type, or type_ in non JS languages.