The job posting is certainly "sponsored content", a privileged post put there at the behest of a third party, which blends in fairly well with neighboring posts yet isn't subject to the normal rules. I don't know if money actually changes hands to put it there (maybe each YC company is granted a posting there, or maybe they have to pay for it), but in other respects it's straightforwardly an ad for a job.

All that said, I have no problem with it being there.