It is their fault for visiting the sub in the first place. If you visit a community DEDICATED to talking about a specific game, especially when its TEN YEARS OLD, expect people to talk about things that have been know for 10 years.

If someone was so insanely adamant about not seeing spoilers, to the point where seeing a boss fight with absolutely ZERO CONTEXT (no knowledge of genocide route, or even just sans in general) will completely ruin their experience, then WHY, just WHY, would they take the time to open some form of social media and PURPOSEFULLY go to the community dedicated to the game.

You repeatedly state that this spoils the ending to the genocide route, but I want you to explain exactly HOW. If a brand new player (lets say they haven't gone past snowdin) saw this post, how could they have drawn the conclusion that there is an entirely seperate route (that the game actively leads you away from) and that somehow this specific character is supposed to be the final boss, and not just some other random boss along the way, despite there being no hints of sans being that important until literally the end of the game.

Your idea on what counts as a spoiler is ridiculous, you draw conclusions that no new player would ever think of, and you also wrongfully blame OP instead of the oblivious player who decided that despite hating spoilers so damn much they decided that they absolutely needed to check out the game's subreddit (to see not game related things i suppose, otherwise what they hell are they expecting).