Your "assuming no repeat customers" is a key point -- when reading overall financial statements like these, note that at "jumbo scale" like Fitbit/GoPro, the "average" customer acquisition cost actually drops tremendously because you get: referral sales / word-of-mouth from existing customers, much more public awareness and perceived legitimacy, more free press coverage, replacement and upgrade sales, gifting from existing owners, etc. all of which contributes to $0 effective CACs on those additional units sold. It does not mean that their
marginal cost to acquire an additional new customer is $6.59; if it were, they'd certainly go out and spend it as fast as they can until saturation.
In contrast, a hardware startup (or a software one, for that matter) just has to go out and hustle to get every single sale, whether through paid or non-paid customer acquisition channels. This is where YC's philosophy of treating hardware and software companies similarly works: they both usually have very similar customer acquisition challenges in rapidly building and scaling their growth channels (rather than, say, technical engineering challenges, which are very rarely the limiting factor in startups).