Seriously I think the investigators know that wrongly calling "Lithium Battery" as the problem would be a
PR disaster so they are careful to call it as an internal OR external issue. Not least as they are probably still uncertain of precisely what DID happen. Remember they spent months not finding conclusive evidence of what caused the main batteries to self destruct even with several to examine
I suspect the "Inert" wording is intended to convey "Make harmless" and thereby encourage the fullest practical set of Battery removal, Cable disconnection and removal/isolation actions