For me it’s a conspiracy theory (and most likely untrue) - but it’s quite an interesting one rather than “stupid”. Raises ideas about authenticity and what people are doing when they go to look at artifacts in museums. There HAVE been paintings, for instance, hung in galleries for years that have turned out to be forgeries but - for a long time people were happy staring at the “art”. Also, ones that have been restored beyond a point where they are pretty much not the original piece. How much would it alter things if the Marbles were all replaced with exact replicas at the British Museum?