Is he confusing REST with AJAX? AJAX was supposed to return HTML but is nowadays often used to return JSON, yes mostly from a REST API.
But I would expect to get well structured data from a REST API. If you return HTML you might as well return it directly suited for presentation which may not be very well suited for data parsing by a computer.
Edit: No, he's not confusing them. The first guy coining the REST term defined them as explicitly returning hypertext. Sounds like every webserver ever to me.