If we're making this about the innocent bystanders now, that's all the more reason to be direct and say "I disagree." rather than indirectly expressing negative feelings (aka being passive aggressive) and asking for a source.
If anything just breezily asking for a source would imply to people who don't know better that this is a rather even keeled take and just needs some more evidence on top. "I disagree and here's why" nips that in the bud directly.
How is "I disagree" any more direct than "I've not heard anything like this. any source that would point at that?" Moreover who's to say this person even disagrees? Personally i don't always ask for them because of a disagreement.
I think the hanging point seems to be that you found the comment passive aggressive but i genuinely didn't.
Right, that's what makes this rabbit hole a bit wild. I'm not even expressing a disagreement, rhetorical or otherwise. What's more, there's nothing wrong with doing that either. There are circumstances where that's a perfectly appropriate thing to do.
And while I fully agree there absolutely is such a thing as smarmy commenters asking for sources in cases where it's misunderstanding something fundamental about the conversation (e.g. "Shakespeare is good", "oh really? source?!") or frivolous requests for factual information familiar to everyone ("global temperatures are rising? Source!?"), I don't know how someone could read this subthread and feel that my question falls into either of those categories.
And to use this of all things as a moment to die on the hill of advocating for fuzzy boundaries between speculation and fact, which absolutely is something that facilitates misinformation, and to be angry that such a thing would be interpreted as a favorable attitude toward misinformation, is completely baffling.
If anything just breezily asking for a source would imply to people who don't know better that this is a rather even keeled take and just needs some more evidence on top. "I disagree and here's why" nips that in the bud directly.