I think they took 1.12 because there are certain open-source projects with readily available source code available publicly.
I remember hearing an excuse some time ago along the lines of "We don't have that source code anymore". Which makes sense, since evolving a product you don't tend to make constant copies and store them forever as older versions of it.
And the bit about 1.12 that makes sense is that there's existing repos of it available. Forking a project and continuing from there sort of makes logical sense to me.
I do wonder often where they got their code though. Reinventing TBC can be a pickle unless you get a readily available source of some certain version of the game. (Something that I'm also after, since I couldn't care less about classic, but BC... now that's a different story).
I could be wrong though.