Why do I feel like this is a case of them already having picked a candidate but still wasting time anyway?
I've seen behind the scenes of these kinds of things, it's possible that they already had a really good candidate but decided to interview everybody in case somebody surprises them and is better than that person. From their perspective (at least what they tell themselves and each other) they "gave you a chance" even though they were pretty sure they already had the better candidate. Whether that's a bullshit reason or not is up for interpretation I guess, in the end for most jobs it boils down to how much they liked your personality, provided you actually have some qualifications for the job.
Being late is mad disrespectful tho fuck them.
Is the job post a requirement for private companies in the US? Never seen that happen in a private company in Europe unless maybe it's like a large multinational one where they need to fudge the internal company paperwork, usually if they want to do a nepotism they just straight up do it.