It's down, as the quoted Hitchcock explanation suggests, to participation. The viewer/player loves to know something the characters in the film/game don't. It gives them a sense of empowerment and increases their engagement, in general. The exception (and the only real one that matters) is the 'monster in the closet' point, that sometimes you do want the visceral reaction of having something jump out at you.
In general though, I'm not sure knowing the plot is what makes or breaks a film or game. If that were the case, why would anyone watch a film twice or why would you play through a game once you'd finished it once? Yet I've watched some films a dozen times and would happily watch them again in future and there are several games I've played through more than once. Jokes I know by heart still make me laugh.
I think the argument I'd come to is - anything which could reasonably be assumed by a generally knowledgeable but specifically naive person viewing it cannot be spoiled. Only something which is genuinely surprising can be. Put it this way, in most movies, the good guys win in the end. Therefore, this can be safely assumed and therefore this
by definition cannot be spoiled. What could be spoiled if this was one of those odd cases where the good guys didn't win because that's going against assumption. Say.. Star Wars. Saying the good guys win in the end, that's not in any realistic sense a spoiler. You'd have be wrong in the head to believe any other outcome was likely. Saying Vader is Luke's father?
That'sa spoiler, because there's no way you could assume that going in.
To me, the point of the article is not that the latter kind of thing is being continually spoiled, or that it wouldn't be rude to do so, it's that some people have now decided anything at all, even the most trivial and obvious detail, is a spoiler and therefore must not be placed in the public domain at any point until they've seen or played whatever it was and they complain publicly and at length when it is. These people are tedious bores and should not be pandered to.