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.