A spoiler is learning about a plot occurrence from an outside source before seeing/reading it in the actual story. When the story itself tells you something, then that's how the author intends you to learn about it.

And that's great that you would still read the last Wheel of Time book if you knew all the details of how it ended, but surely you don't think you speak for everyone, and that's exactly my point. Not everyone is okay with spoilers. It's a very individual and case-by-case thing, and so there is no one size fits all solution except the status quo, where spoilers are marked and people can either choose to read them or look the other way.