You've already been given several reasons why they might do that:
1. They don't want users to buy old board with old bios, fit a new CPU and then complain it doesn't work.
2. They're lazy.
3. They forgot.
4. They're just listing the board how it was originally sold.
We've seen it MANY times with motherboards where the specs don't get updated. It is far, far more common than the board has no support.
If you think you know everything and we're all idiots, I don't know why you bother to post here, go start a YouTube channel and tells everybody else your FACTS.
The AGESA updates are what define CPU support, but like I said, you can check forums like reddit for actual user experiences with these boards (which some of us have already done for you).
I can't imagine Gigabyte will update the BIOS with an AGESA that has support and then deliberately strip out the 9000 series microcodes, but the only way to confirm that yourself is to check within the BIOS.