The version limits come from the fact that the Nix cache doesn't maintain older versions. So, if you use an older version, you will have to compile from sources. It sounds like they didn't want to take it upon themselves and provide a cache with older versions, even though it doesn't sound like much effort.

Honestly, the reasons given don't feel very solid. Maybe the person who introduced Nix left and the ones remaining didn't like it very much (the language itself is not very nice, the docs weren't great either in the past).

Still, I'm not familiar enough with the stack they chose, but does it provide a level of determinism close to Nix? If not, it might come to bite them or make their life harder later on.