I don't know why you'd be surprised by this. The top kernel on kernel.org's front page is literally just Linus's personal tree. "Upstream" is context-sensitive but "mainline" has always been Linus's personal tree.
In reality, what matters to most users is what goes in to Debian stable or RHEL, and those kernels are far from just "works for me" tested.
Linux has always been treated like it's Linus's personal project. That's why more than a few of us believed that Linus taking a 'break' a few months ago was the beginning of the end. It turns out that not much changed, but the culture is still there.
I don't know why you'd be surprised by this. The top kernel on kernel.org's front page is literally just Linus's personal tree. "Upstream" is context-sensitive but "mainline" has always been Linus's personal tree.
In reality, what matters to most users is what goes in to Debian stable or RHEL, and those kernels are far from just "works for me" tested.