It's in the fact that all these complaints were vague and lacked details. It's tempting to think "all these unconnected people said the same thing, so it must be true" but those people are not unconnected -- at the least they're connected by prevailing culture, which can be very racist while being understated.

And the thing is, that's orthagonal to whether or not the receptionist was actually rude (LW also gives no details, but it *has* been years). Bigotry doesn't only affect complete innocents. I can even see (from having lived it) a nasty feedback loop where the receptionist was as kind as she could be but people treated her terribly so she gave up and became surly. Bigotry can wear one down and curdle one's soul.