Given how late the review is versus most journalism purveyors, I suspect they bought their phones and used them for a while before submitting the review.
Also they tend to be upfront about when they have a potential conflict of interest (often in dealings with car manufacturers), or for example when calling out issues with some other company that their parent, Conde Nast, has an interest in. It's not like they can't afford to buy most of the products they review, either.
That said, explicit statement of said assumptions would be welcome, but they're one of the few sites I actually trust.
I'm a bit disappointed there was little to no comment on the antenna controversy, but Ars tends to only publish articles when there's enough evidence to talk about (by its nature, Macrumors comments on rumours). I had a quick Google and they wrote up articles on the iPhone 4 antenna within a week of release, and whilst we're still early in the XS release window, it seems like it's less reproducible so far, which is good for those considering a purchase.