“Departmental rigor/reputation is more important/narrow in philosophy than in may STEM subjects. In the US, that means that a top philosophy PhD program will be chock full of Ivy/equivalent/near-Ivy grads with almost no representation from “regular” schools besides Rutgers and Pitt (which are 2 of the top unis in philosophy).”
This is not true in the US. Many top students for undergraduate go to state schools or schools below what they could get into that offer scholarships. Probably most humanities graduates from top 20 schools go to law or MBA school, not graduate school in humanities. Not sure the reputation of the department is that important in graduate admissions either.
Top US law, medical, and MBA programs definitely do have mostly students from top undergraduate schools.