Its not, Saber Marshall's robe come only from one source, the Revanite Quest line.
There is a few version of that chest in-game:
- PvP vendor 1-49, White with blue i think.
- Crafted by Synthweaver, Black with an orange stripe on the head.
- Purple world drop pre-15 (same as the one in the Cartel Market), White with an orange stripe on the head.
I never saw it drop myself, but I quite remember seeing the robe for around 1 mill on the GTN 9-10 months ago.
Since the one you get as Revanite quest reward BoP, its logical that at the very least it used to be a world drop.