I would add to the definition of a spoiler, that the future detail has to have a degree of specificity. How specific depends on who you ask. Saying something like "the next chapter is going to be about slice of life" is probably not specific enough to be a spoiler. Saying the protagonist will lose the ongoing battle 5 chapters from now is pretty specific.
Also, spoilers are from people who know what will happen, usually because they read ahead in the raws, or because someone else told them. An honest theory about future plot can sometimes be misunderstood as a spoiler if the theory turns out to be accurate. And sometimes readers may disguise a spoiler as a theory, but it would be difficult to prove that.