I would imagine it's more you're being skeptical of something that is unpopular to be skeptical about. It's like someone saying climate change is impacting our planet, and then asking "source?" in response.
No, that's not correct. I ask "Source?" when someone makes a claim that goes against popular belief, such as: "climate change is not impacting our planet." I do think "Source?" is generally considered a low-effort response, so it's the wording I guess, not the context.
He was asking about the trend of him commenting with "Source?" leading to downvotes on other posts. I was giving an extreme example to represent why that may be happening elsewhere. I agree that in this case with the 3% rate it's likely not applicable.