John Fetterman tweeted a picture of fictional dad Homer Simpson in response to conspiracy theorists who just won't stop believing he uses a body double.
You seem to be badly misunderstanding this expression. Just because, once one has made a first impression, they cannot change that first impression (any more than we can change any other historical event), does not mean that the impression is accurate. That's really the entire point of the expression - that, if you aren't careful about your first impression, you may be judged unfairly, by people who are not wise enough to look deeper. Using that as justification for refusing to look deeper is absurd.
I know exactly what it means, just like yes you can judge a book by it's cover. That doesn't mean it's a correct judgement, but it certainly means the author/publisher needs to present an attractive cover.