OPPO announcing doesn't means push immediately
You did click on the links and read the articles right?
"ColorOS 12 will be available on the Oppo Find X3 at launch" The Find X3 is a phone that is already in circulation, so that says the OS will be released for that phone on that day. Likewise for the earlier announcement on the Realme GT getting RealmeUI 3.0 (based on ColorOS12, which is built on Android 12). You can expect to see more of the phones under BBK's stable (
Oppo, Vivo, OnePlus,
Realme and iQOO) to follow suit.
First to market the latest OS has a strong psychological advantage when it comes to consumers. But more importantly, if those Chinese phones beat Google to the market with Google's very own OS, and people know that they also need additional time and effort to skin it and all, it reflects really badly on Google themselves - already the announcement shows us in-house issues. It sends the message that Google does not want to prioritise support on its own phones, which it does not need to skin or customise unlike other OEMs. Why would a consumer continue to buy Google Pixels if they can't get the latest updates/OS before other Chinese phones which are cheaper and better spec'd?
If the reason to hold back was, as cited by
@kakaBoBo earlier, to give the Pixel 6 some kind of "new flavour" feel, then letting two competitors (cos they are different business entities even though they are owned by the same umbrella corporation) steal that thunder is worse than letting their own Pixels do it.