Nobody said you can't talk about something just because it's old. You can talk about whatever you like, of any age, but don't jump to the conclusion that because it's old, everyone has seen/read it and knows what happens.
To be frank, you said "spoilers, lol" at the end of your post, which indicates you knew exactly what you were doing. It takes no effort, even when you aren't aware of the fact that we have spoiler tags here, to rearrange that sentence and put "spoilers", y'know, before the spoiler.
It seems like you've learned for the future, though, which is a good thing. Your attitude about it remains alarming, though. A simple "sorry" and editing of the post would suffice -- not starting an argument that it's everyone else's fault.