I have no idea what will happen for the gen 2 Zs, I don't even own a Nikon (yet), but I find it logical that the bodies with the less good AF are updated first. And it's actually quite admirable from Nikon to provide such a significant improvement to bodies 2 (well 3 if you count on the Z 9 is the third gen in term of AF) generations back!
If I understand your post correctly, the Z6/7 gen 1&2 share the same processor, and that might be the limiting factor in updating gen 2 ? They might already hitting their processor limits?
I wish that sort of customer care was more common. We all know quite a lot of camera bodies could perform better than they do with some firmware love, but it rarely happens intentionally to sell a newer body. I'm not saying every camera could be 2x better, but certainly quite a few things could be marginally or significantly improved and aren't. Let's hope this great firmware update thing from Nikon never stops and inspire more brands to do the same. In fact, coming from software development (not as a developer), I would find perfectly acceptable that firmware could be paid when they bring a significant improvement over the original body you paid for.