How do they know it's a "nature-replicating experiment"? Nobody was there to observe the conditions under which, in theory, viruses spontaneously generated from some non-living substance. Circular reasoning.
> Nobody was there to observe the conditions...
Pre-biotic conditions on Earth cannot be known with certainty, but they can be known *generally* based on conditions foudn in the rest of the universe. Free oxygen, for example, is astonishingly rare. Somewhere around 95% of the visible mass of the universe is hydrogen; thus hydrogen dominates. Free oxygen very rapidly chemicaly bonds with hydrogen and forms water. Same with methane. And there are no known non-biological processes which will both produce and sustain an oxidizing atmosphere. If you were to kill off *all* life on Earth, Earth would have a carbon dioxide/nitrogen atmosphere relatively soon.
Looking at the rest of the universe, we can make a reasonable approximation of wha tthe pre-biotic atmosphere of the Earth was like. It it not "circular reasoning" to conclude that what holds true most places in the universe would also hold true on Earth. Consequently, a CO2/methane/nitrogen atmosphere on the early Earth is entirely reasonable and consistent with all available information.