Many artifacts were stolen from legitimate kingdoms / village councils / etc that no longer exist, so for these artifacts returning it to the corresponding modern government is fairly appropriate.

Otherwise, unless there is a clear claim of individual/familial ownership vis descendancy, then returning artifacts to a legal national government is still a least bad option. In this case the artifact belongs to the people and is stewarded by the peoples' government - the government doesn't "own" it in the way King Charles owns the Crown Jewels. (Ideally this government would be democratic, but international legitimacy should be enough for the UK to hand over the goods.)

There are a ton of exceptions - Rohingya artifacts shouldn't be sent blindly into Myanmar - but I promise the people involved are taking this seriously. It seems condescending and arrogant van Bommel really failed to "think through" her usage of a metonymy.