I mean, a good chunk of caution is warranted, but this matter of fact (heh) statement without any supporting evidence isn't really helpful either. Particularly because this isn't just Google, but all major players in the smart home area: Apple, Amazon, Google, Samsung and the Zigbee Alliance. I heavily doubt that any of the other players would freely give Google any sort of access.
Well, technically nothing implements the pairing mechanism entirely right now so yeah, at least the pairing still phones home. I believe homeassistant requires you to use google home to pair the device to it (though that may change, the extension is still in beta)
Interesting. As far as I can see you can pair Matter devices either through the HA app itself, Apple Home or Google Home, so I don't think it's required. I'll keep that in mind though before I get my first Matter device.
Last time I checked the HA app uses the system tool, which just uses Google Home behind the scenes. Apple Home is probably the only other implementation of the pairing tool.