There's some miscommunication / misunderstanding going on with your supplier.
FCAW electrodes comes in two basic types, one that requires shielding gas (through the weld gun to protect the arc) and another that does not.
This has nothing to do with internal purge gas on SS pipe.
If you can do an open root with FCAW or SMAW on Stainless, then you could possibly have a fully shielded root pass on the back side. This is possible but not easy. I have qualified with open root SMAW on stainless before for window welds and its very fitup/amperage/technique critical.
For Stainless FCAW, I am sure the possibility exists to do the same thing but that does NOT change the requirements of your document. If your supplier has some open root Stainless FCAW skills, I suggest asking for a demo!