"I don’t have contact information, and like I said, we’ve never communicated.
Do they really expect me to dig up his number after he abandoned us 18 years ago and ask for his help?
That seems odd, given that he is a literal stranger"
Not as odd as you might think. For schools that request/require NCP info, they often think that lack of contact or of a good relationship simply does not excuse him from they consider to be his ethical responsibility as your parent.
So if you know who he is or how to contact him, they expect you to try.
They also think that college costs are not exactly unforeseeable, and therefore you are expected to do your best to patch up relations with him well before filling out college financial aid applications. If you and your custodial parent have consistently made efforts to reach out to him (and can verify that), that’s one thing; if you and your custodial parent maintained the isolation from your side, that’s another (not good) thing.