Fielding's thesis barely mentions HTML (20 times), and usually in the context of discussing standards or why JS beat Java applets, but he discusses hypermedia quite a bit (over 70 times).

If you extended JSON so that URLs (or URIs) were first-class, something like:

    url ::= "<" scheme ":" ["//" authority] path ["?" query] ["#" fragment] ">"

it would form a viable hypermedia format because then you can reliably distinguish references from other forms of data. I think the only reason something like this wasn't done is that Crockford wanted JSON to be easily parsable by existing JS interpreters.

You can workaround this with JSON schema to some extent, where the schema identifies which strings are URLs, but that's just way more cumbersome than the distinction being made right in the format.