I think most of the confusuion comes from "AT TIME ZONE" being the syntax for both the conversion from _and_ to timezone'd timestamps. Maybe it would have been more intuitive had the two operations gotten different wordings, e.g.:
timestamp to timestamptz: AS ZONED AT TIME ZONE ...
timestamptz to timestamp: AS LOCAL AT TIME ZONE ...
Felk · · focus · HN ↗
timestamp to timestamptz: AS ZONED AT TIME ZONE ...
timestamptz to timestamp: AS LOCAL AT TIME ZONE ...