You're wrong. The author is using "REST" to mean an API at Level 3 on the Richardson Maturity Model[0] - this was the original conception of what it meant to be a "REST API" before the wider internet decided "REST" meant "nice looking URLs". What he refers to as "OpenAPI APIs" could be called Level 2 Web APIs on the same model.
He uses "REST" correctly. He uses "OpenAPI" as a shorthand for the class of web APIs that are resource-based and use HTTP verbs to interact with these resources.
I could concede perhaps he wasn't necessarily wrong on REST, though I personally think it's pedantic and incorrect, regardless of what the creator of the term says. Things evolve, and returning a list of objects instead of a list of links was an obvious progression, since spamming 1000s of GET requests doesn't scale well in the post 90s. If the industry at large generally agrees on what makes an API restful, it feels like we should accept such evolution.
OpenAPI is a description language and has little to do with an API itself. It's documentation. People were using 'unpure' REST long before it or Swagger even existed. And as the parent pointed out, you can add an openapi spec later, and it doesn't magically change the API itself.
Further, it creates a weird circular logic that doesn't work.
