I think in addition to a questionable attitude, he has a point.
Harry Potter is legitimate in having 7 books, because it first appeared that way. 8 movies on the other hand was a problem for some due to the fear of it being stretched thin.
Same thing with The Hobbit - small book, perfectly fine. 3 movies however were reason for concern.
If FF13 came out in one, it would still very much feel like 3 different games, because of different themes, story, game world design and gameplay.
And I think it is a valid concern to have for FF7 to feel like one third of a game. If they make it into a very different game (and gameplay wise and graphically they already did), part 1 could feel like a whole thing. Bit it is a remake and I suspect the story not to change too dramatically.
You are right - remains to be seen if part 1 can stand on its own. But I am sceptical. And I think for good reason.