Weird, I wonder if this is some sort of phased rollout so they minimize risk of something goes wrong. Major HomePod OS updates haven’t always been Apple’s smoothest OS releases.
I have multiple HomePod minis, the update appeared after a few hours for me. I have had the same issue with previous HomePod updates, checking for updates does nothing for a while and then they eventually catch up.
At least it’s not as hard as trying to summon an AirPods update.