They can't use a current build, as they'd have to modify it to turn off a bunch of things (Abyssea, Adoulin, the QoL improvements in the past year, new job traits and abilities, NPCs related to level 76+ content), which would make a new build, which would have new bugs. Old builds have bugs they've fixed with more recent patches. Reverting to a previous build would reintroduce those bugs.

Also, the game client and server need to identify as matching versions. Notice that the test server required a seperate install.