"5 man AND pre-nerf"
It is only my guess, but maybe that they use the "first" 5-man version, which is not always 1.12.
Imagine that Scholomance was 10 man at launch, was nerfed to 5 man at version 1.6, and nerfed again at version 1.11 to make it a little easier.
Then, Nostalrius would use version 1.6, the "pre-nerf 5 man", and it will not change over time, except for bug fixes.
(I used fantasy version numbers, it is just an example.)
It is the same for Alterac Valley. It is in "pre-nerf" version, not 1.12, and it will remain in that version all along the server life.
It was the server's philosophy when it was in french version. That was pretty immersive, and I had the best "blizz-like" feeling I ever had on a private server