I'm not sure that this ability to be firewalled is what keeps Homekit down. Inexpensive Matter devices abound that can also be firewalled from the internet, and never connected to it even once[1].

[1]: Yes, I know. There exists Matter-upgradeable widgets that must be connected to the Internet before becoming usable as Matter devices; send those back. I'm also aware that there's Matter devices that require Internet in order to be useful[2], but every instance I've seen of that is just something that was designed by a moron, imbecile, or idiot; send those back, too. Returns cost these companies money.

[2]: https://www.reddit.com/r/TrySwitchBot/comments/1j8ogt8/matte...