Why are my HomePod Minis sending ICMPv6 router advertisements?

No, nobody but me, secured WiFi, no guest LAN, etc.

One or more of the following seems to have helped, if perhaps not yet 100%:

• manual IPv6 configuration on the affected laptop, using values that automatic configuration (when working) would have used (a few days ago - didn't help much)
• a pair of rules added to pf.conf, one to pass router advertisements from the actual router, another to block all other router advertisements (may have helped, esp after reboot to get it in early before things got crazy)
• HomePods (regular and Mini alike) updated to 14.4 (most recent change)

A Mac Mini running Mojave connected via Ethernet (same LAN as the WiFi, nominally) had less or no trouble, ditto systems running Solaris 9, 10, and 11. Windows has some IPv6 issues occasionally, but that's partly that 2 of 3 are VMs, and also that the router occasionally (weeks or so, not hours or minutes) needs to be smacked, to be told to release and renew its external IPv6 address.

It's livable now, close perhaps to how it was before I got the HomePod Minis. But given that my router is NOT Thread-aware, and I have no Thread-capable devices other than the HomePod Minis themselves, there's really nothing for the Thread-capable HomePod Minis to be routing to or from, so I still don't understand why they send router advertisements.

I see something I can download that basically should let me explore Thread functionality nearby, so maybe if I can get that working, I can see whether some neighbor has a Thread device that's confusing my Minis. but it's supposed to be shorter range than WiFi, so that's not a very plausible theory either.

I have iHome smart plugs, which don't depend on that (they can be accessed either as HomeKit supporting WiFi devices, or via Amazon Alexa, maybe also via IFTTT), and Philips Hue bulbs and hub. The hub is where the HomeKit comes in, and it supports regular WiFi (not needing Thread either), the bulbs are Zigbee, but they're controlled via the hub. All of that was there before the HomePod Minis.

Unrelated (maybe) to the router advertisement problem, the latest HomePod update seems to have helped out with the iHome devices often being unresponsive via HomeKit (which also I think started with the Minis), although still responsive via their own app. So maybe that update helped take care of both problems.