The value of YC as an investor is not the cash they offer but the advice and guidance. As such their influence is disproportionate to their investment.

They likely do have the power to disassociate themselves from Airbnb though.

(Like, I don't think that Y Combinator owes me anything or gives a shit about what I think or whatever, I just think "they are a minority owner" is weak sauce)

I would say that YC has a relatively large cultural impact, given its investment size. People outside of SV barely even know who Sequoia is, let alone care about who they fund. But a lot of people know about YC and what they are doing. While financially, YC pulling funding wouldn't have a big impact, culturally it'd be a huge bomb.