> It's "closed" in the same way that all open wireless standards (Wi-Fi, Bluetooth, etc.) are closed. You can read the spec, use the open source SDK, and build devices without paying a cent.
My understanding - correct me if I'm wrong - is that it's not quite the same; I'm pretty sure you can make a wifi card on your own (maybe modulo FCC approval, but that's true of any radio) and you might not be allowed to put an official wifi logo on it without a license but it'll work without needing to see an officially signed cert or the user having to touch developer settings.
You can definitely add uncertified accessories (using CSA test Vendor ID 0xFFF1) to HomeKit via an "Add Anyway" confirmation. Because Apple tends to be extremely conservative about this kind of stuff, I'd expect that all systems that support Matter would allow this.
My understanding - correct me if I'm wrong - is that it's not quite the same; I'm pretty sure you can make a wifi card on your own (maybe modulo FCC approval, but that's true of any radio) and you might not be allowed to put an official wifi logo on it without a license but it'll work without needing to see an officially signed cert or the user having to touch developer settings.