This means the source code is finally being released for the quarterly release that came out in september. Roms like lineageos had to target QPR0 which came out back in June but can now bring up to this. Google used to release the source to AOSP right after the releases happened, now they don't.

Additional context per fediverse thread: The GPL code (i.e. kernel) was released on time, this is the AOSP userspace portions which Google isn't legally obligated to release (which doesn't make it not a dick move not to).