Perhaps you'd be surprised in knowing that a resource can be a stored procedure, quoting Roy Fielding[1]:
"a single resource can be the equivalent of a database stored procedure, with the power to abstract state changes over any number of storage items"
In general I think PostgREST's REST implementation is evolving. Also, we've had Roy Fielding giving feedback[2] on an issue before. Once we fix that issue, we'll ask him if he thinks if PostgREST is REST. I have a feeling that he might reply positively :-]. We'll see.
[1]:https://roy.gbiv.com/untangled/2008/rest-apis-must-be-hypert...
[2]: https://github.com/PostgREST/postgrest/issues/1089#issuecomm...