Internally they are with Pg. In practice non-TZ columns can go sideways. For example your app or ORM starts trying to store local time (Pg cannot know to save equivalent UTC), so now you've no way to correct and Pg cannot either. Or the DB config gets switched to non-UTC, again you may have no recourse.
I've seen both these things happen at companies. Often users don't care or notice for a long time, until suddenly they do and you're painted into a corner.
I understood timestamps to be "seconds since 0:00 UTC January 1 1970", so absolute points in time that are not timezone-dependent. Wikipedia agrees with us too. Of course there is no official definition, and the term has evolved from an older concept (actual rubber stamps) so it's not surprising that others use it differently...
I've seen both these things happen at companies. Often users don't care or notice for a long time, until suddenly they do and you're painted into a corner.