Who remembers where this comment was recently from a Tesla employee commenting on the challenge/need of merging AP1/AP2 codebases?
I think it was just misunderstanding.
What they really meant is there's a common codebase for everything from S to Truck (some bits on a side branches that are periodically merged with mainline - to avoid too much stuff leaking). Now there are compat modules so that the UI can talk to different kinds of hardware, be it different autopilots or body controllers or whatever.
So every time they add a new something there's a compatibility check in the code, but it does not mean they port latest ap2 firmware onto the pre-AP-style rain sensor because that makes no sense, but since firmware is mostly monolitic, they do carry around firmwares for all possible modules even if your particular car does not have that module, it's just not used in that case.
I did not look into AP1 firmware myself, but reportedly it did not change since Oct 2016 or some such, all the improvements people are seeing is due to changes outside of AP1.