Three words: connections, connections, connections. Especially since this is still in academics, if you don’t know someone’s letter writers, there’s probably no more than a 2 person chain of phone calls to find someone who does. I could rattle off a list of names on a good letter that will get you, bare minimum, a strong chance for an interview anywhere.
The “don’t know them” rule is more common with applicants from schools with lower ranked or out-of-region departments where you have to figure out who the LOR author is. If you know and are comfortable with the mentor, you more or less know the applicant. It’s also more common with lower “prestige” departments because they worry if the superstar applicant is at all interested in going there. Yes, your PD and the residents help (with your PD and other attendings being exponentially stronger). Your away PD? Only at that school because you probably didn’t work with them for more than a week. Rank lists are made with a panel, and residents have some influence and usually a certain number of votes to cast, but it’s a substantial minority to attending votes and influence. Where residents do have power is telling the attendings you’re a bad fit, and then you’re sunk.