For E3 it's really not hard to figure out why people classify game reveal leaks meant for E3 as a spoiler.
Just think of an E3 press conference like a movie. A game reveal is part of that movie. Knowing that's happening is like someone telling you what's happening at a particular part of a movie.
It's a bummer when it happens and does hurt the experience for some people. It's in no way "unhealthy" to feel that way and Schrier is wrong for saying so.
I mean, Sony's conference from a few years ago is looked back so fondly upon because of the surprises. The surprise of Shenmue 3 and the FFVII remake were so much more fun to experience without knowing they existed before hand.
In the end, it's all marketing, sure, but so what? People should feel bad for getting excited about a product?
Hey now, what I learned is that not everything in a movie is a spoiler. Only the more twisty and important plot reveals and developments are.
That's kinda the problem right there. Movies, games, books, etc. have spoilers because they have a story. E3 doesn't have a story, it's just a bunch of announcements.