HTML is a hypertext.
Yes, Github probably has one of the most as-REST-like-as-possible JSON apis, and it's mostly wasted effort. It would be just as usable, as an API, without all that stuff, since it is consumed by code, not a browser/human.
And a bunch of things called "browsers" that implement that spec. More or less.
I mean, come to that, aren't XML and HTML both just text? Isn't text, then, a hypertext? One can embed URLs in text, after all...
JSON as JSON isn't a hypertext in the same way that XML as XML isn't a hypertext. XHTML is a hypertext based on XML. (HTML itself isn't proper XML.)
There are hypertext specs based on JSON, such as: https://json-schema.org/draft-04/json-schema-hypermedia.html
But, without clients that treat the data as a uniform interface, it's all mostly pointless, which is why most JSON APIs stop at Level 2 of the Richardson Maturity Model, and why things like GraphQL are becoming more and more popular.
In the original model of the web (which is what Fielding was describing w/ REST) where HTML was being delivered to a browser, the uniform interface works: a browser has no idea what the HTML document means but can render it to a form that the human looking at it can make sense of.
And, have you seen Github's API implementation? They have the links and self-referential details in their particular json schemas that you seem to imply are necessary and unique for that.
If the argument is that json can't be rendered, isn't it arguable that xml equally has that problem, we just have a program with a common syntax reader format that renders pages equivalently, and something equivalent could be written for a particular json schema?