Okay, so, it seems you haven't pieced together the thing you want to know from the many chances you've been given from context of what everyone's said, so I am going to be extremely specific about what constitutes a spoiler, and why people get upset if you give it to them:
You have a movie called the Titanic. Okay. Everyone knows the ship sinks -- well, for the most part it's common knowledge, but there are always some people who are too young to know, or have missed a certain fact, anyway, for the purposes of this example, assume the majority of people know the ship sinks -- so the fact that the Titanic sinks is in no way a dramatic twist. And because it's not a dramatic twist, it cannot be a spoiler. I'll define a spoiler now: A spoiler is any event inside a narrative which the author of the dramatic piece wants to use to illicit shock from an audience. In the majority of narrative works, telling someone that "the hero gets the girl at the end" is not actually a spoiler -- even though some will get mad if you say it -- because it's not a twist. That's what usually happens. Like, 90% of the time if a movie maker is going to formula, the guy gets the girl is a beat of that formula they will hit. So why am I bringing this up? Because "the guy dies in the freezing ocean while the girl makes it safety" is a spoiler! It's a dramatic twist meant to illicit an emotional reaction, and it works best on the majority of audiences as a surprise. This, of course, means that there are outliers who have no idea why people get upset at spoilers, and it's because they do not share the same neural make-up as the intended audience of a piece. However, if this does not actually describe you, I'll keep going.
The spoiler cannot come from any piece of the actual historical record. Whenever an author is working with a particular historical context, there's always room to add original characters for the sake of the drama. That's why everyone's here, the drama. The story. The historical event is just a structure to build the story you want to tell around. Stories are essentially a tool to transfer emotional states and possibly knowledge between one person and a lot of other people. The largest impact a story will have on you is the first time you hear it. The reason modern audiences have grown so picky about spoilers is because we are only recently living in a golden age of story telling where there is actually no need to hear the same story twice. We used to have it so bad. We used to have to hear the exact same story over and over until we had it memorised. But now, now we get to experience the emotional hit of being surprised by a story whenever we like! So yes, the modern audience has gotten picky about spoilers, because to hear the supposed-to-be-surprising beats out of context will, for the majority of people, take away from the emotional impact that the story could have had on them.
That's why people care about spoilers.
And if you don't understand it after this explanation, I honestly have no idea what to tell you.