You've said this in a few different places, do you have a source for that? You are saying you could load AOSP on an S9 regardless of what Samsung does?
If so, this is indeed huge, but I have some serious doubts about it, perhaps because I have felt so very burned by Android so many times in this respect on anything but Nexus devices.
Maybe - the bootloader must allow you to run that AOSP ROM, which only is possible if the bootloader isn't locked. Lots of OEM:s lock their bootloaders. But otherwise, yes, there's nothing else stopping you.
There are a bunch of things potentially stopping you in practice. The HAL isn't a total wall, and I expect to see many OEMs go around it in some places, either due to incompetence, the HAL being inflexible, or timescales.
So even in situations where you can install such a ROM, there's very little guarantee key bits of hardware will operate properly. Any non-standard hardware/hardware feature is also fairly likely to break.
If so, this is indeed huge, but I have some serious doubts about it, perhaps because I have felt so very burned by Android so many times in this respect on anything but Nexus devices.