All of that's talking about the userspace though?

Not all. For example kCFI is kernel space.

Also, attack surface reduction is a very valid strategy, so it may seem like about the userspace (sandbox for every apps etc) but it could make a big different in how much of the kernel attack surface is exposed.

Yes, but the concept of CFI is only mentioned in passing in that entire thread, and the kCFI implementation used is a vanilla kernel feature and not android specific.

There's a lot to be said that "Distro kernel config choices may not be as secure as possible", but that's not really an "Android"/"Vanilla Linux Kernel" difference.

Well, I don't know kCFI being enabled on any distro besides Android, cause it requires building the kernel with Clang.

The previous in-kernel CFI implementation (before the kinda joint effort - kCFI) was upstreamed by Google, too: https://www.phoronix.com/news/Clang-CFI-Linux-Patches and https://www.phoronix.com/news/Linux-Kernel-Clang-LTO-Patches. Pixel devices also had this long before. Given that the entire Linux kernel feature was developed out of Android I find it a little bit unfair to call it "using a vanilla kernel feature".

I'd argue that the entire point of using a shared open source kernel is that other users can benefit from additions.

Arguing "Who first added a feature" seems to be a losing spiral of needless tribalism. How many features does the Android kernel use that weren't developed by Google? Does that mean they wouldn't have developed those features? Or just that there's no point making a parallel implementation if there's already one existing.

The point here is not who first added the feature to Linux kernel. The point is Android cared about security, built a CFI implementation, started shipping it back in 2018, while Linux had other priorities and didn't have it until 2021. And even then almost nobody adopted it.