If I tell you the sky is green the best way to refute that to your audience is to ignore me entirely as a person and instead tell people to go outside and look up: Show them some evidence and factually refute my claim. You don’t say “this man isn’t a sky expert, has never spoken to sky experts, and should therefore be dismissed!”

The parent comment’s point would seem to be that this person is making a claim that should be refutable with evidence. But the evidence is Facebook’s own data supporting her claim so they can’t do that. Instead, they diminish her credibility. It’s not “smearing” in the sense of calling her a baby eater, but it is a credentials fallacy meant to make people dismiss her claims regardless of validity.