Yes, I'm aware that "legacy" is a common term used like that. However its definition in that glossary, as edited by the game's developers, implies to me that it was meant as a name in this case. It specifically says that the term refers to the old language:
Legacy This term simply refers to the in-editor scripting language used in previous Bethesda Game Studios editors such as the GECK and Construction Set.
In any case, taking into account that the language was used in the Fallout games as well as Oblivion, I don't think ObScript, TESScript or other variations are entirely appropriate.
That said, those terms are perfectly recognisable.
Cipscis