You linked to AOSP, not Android. AOSP doesn't have things like the Google Play Store; the shipped image is substantially different and isn't necessarily open-source just because it contains open-source components - Windows contains BSD networking code but that doesn't make Windows open-source.

Sure, but the vast majority of those mods and alternative distributions are based on the sources that are available.

It is a bit like a Linux distro running Steam. Yeah, there are proprietary bits, but the FOSS part naturally happens on the FOSS parts.

You know what the "A" in AOSP stands for, right? :-)

All I'm saying is that there's a big difference in jailbreaking an OS when most of the source code of that OS is publicly available for study and experiment. Surely that is not a controversial statement?