I think you missed his sarcasm.
I agree with you; possession is nine points of the law. The British Museum has had clear possession of these artifacts for 200 years. That's more than long enough to give them title. Nither the two govts. that are trying to repossess them even existed when the BM acquired them.
There is a statue of limitations of 100 years in archaeology, but I'm not sure how it would apply in these cases.