sounds like the page should say "at least eight years" then, instead of "up to eight years"

Correct. We've just updated the blog post with the proper description from Google, which is "automatic updates through June 2030."

No because that is based on date of release not of purchase.

If you purchase a brand new chromebook whose model has been sold for 3 years already you won't get 8 years of support.

A specific date would be better, because otherwise it’s ambiguous. Eight years starting when?

Google provides specific support dates on its Pixel and Chromebook devices. For instance, under "About ChromeOS", mine says, "This device will get automatic software and security updates until 2027."

https://support.google.com/chrome/a/answer/6220366?visit_id=...

What's more likely, Google break their commitment or they provide extra patches past their commitment.

In case of doubt you can always replace the OS with whatever Linux distribution you choose, which only leaves closed-source driver/fw blobs - but that problem is shared with every other general-purpose computer these days.

Is that after release or after the last sale of the model?

With Chromebooks, that would be after release.