REST, including HATEOAS, was largely retrospective documentation of the architectural underpinning of the WWW by Roy Fielding (who played an important role in web technology from 1993 on, was the co-lead for the HTTP/1.0 spec, the lead for the original HTTP/1.1 spec, and also, IIRC, lead or co-lead on the original URL spec. The things it documented existed before it documented them.