At the best of times, what exactly does peer review add? I'll buy that it catches glaring flaws in their methodology or conclusions drawn from data. But it's not going to catch little mistakes or unlikely events that might have contaminated the sample.
For the purposes of a site like this, peer review filters out the cranks. Cranks can be rather a burden on people: they rarely accept criticism well, and effort spent explaining the problems to their defenders is a drain without a lot of upside.
So when somebody can say "It has been peer reviewed", it at least helps get past the crank filter. And conversely, "It hasn't been peer reviewed" can be a shortcut for "I'm not going to put a lot of effort into this without better reason than I've heard so far."
It's not so much an argument as a way of shortcutting an argument. Though note that doesn't really apply to a top-level comment on HN, where nobody has really advanced an argument yet and simply ignoring it might be a more useful strategy.
there has to be independently reproducable results from independent research over time until the variation of outcomes is known, for some sciences this can be a very long process for very unscientific reasons.
one thing not addressed is that the isotopic elements of the meteoric fragment may have been incorporated by a terrestrial biological or chemical process
whenever the methodologies of a scientific experiment are repeated the underlying principle is preserved if there is no signifigant deviation of observed results from expected results.
for example PCR has a lot of credit, the central dogma of molecular genetics has a lot of credit, but there are exceptions being found in very niche cases such as reverse transcriptase and the HIV virus.
those experimental protocols that have been reviewed and found to be non deficient at the time of review are often repeated in principle as they are used for a common tool.