Nix tends to have updates almost as quickly as Arch does, I run a lot of bleeding edge software and have no issues, and the breadth of packages is immense. Nix just has a different style. With the AUR, you can adopt an abandoned package more easily. That has pros and cons (the AUR has recently suffered multiple supply-chain attacks due to packages being adopted and infected).
I'm fairly certain that the graph you're thinking of is complete bullshit. It only had approximately a third of the package count listed for a particular Ubuntu version than said version had in actuality.
Source?