Theory or no theory, the point is that, at the absolute least, you're saying "in the first five books worth of stuff we aren't told Jon's mother and there's at least some measure of evidence to suggest Ned isn't his father". Do you not see how that's a spoiler?
Actually, not the case. The latter isn't a spoiler because that evidence is already present in both the show and the book. The former doesn't work either; the theory could compete against various "revelations" just as easily as it could work against total silence on the matter. That's the nature of fan theories; they extrapolate from limited information or they seek to find and fill holes in the current "official story."
Beyond that, you're saying "because some other people who don't know the book theory managed to flukily guess it, no-one should be annoyed if I strongly hint it in front of them. If someone managed to guess the Red Wedding in a separate topic, would you feel content to come in this topic and be like "well, if I was Robb I'd probably steer clear of Walder Frey's place, could be bad for him, wink wink, nudge nudge"? No, of course not 'cause that'd be a dick move.
But I'm not strongly hinting at anything because there is nothing to hint at. It's just an unproven fan theory dating back to the first book and the first episodes of the show's first season. There is no comparison to be made one individual flukily guessing future twists and a large mass of people independently considering very common theories that have been around from the beginning.
And don't say "it's only a theory", that doesn't make it better - either the theory is wrong and you're getting people riled up and misleading them for no reason, or the theory is right and you're spoiling what would be an awesome plot twist if you weren't expecting it.
This statement is nonsensical. If it's true, than literally all discussion of the future must be banned because it could "ruin plot twists," whether that discussion came from me or any random show-watcher.
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SubDeity wants to vote for Calvin Coolidge. [Evil Republican]
Play Der Langrisser.