a better question is "how" is this closed source?!? It's linux based, so there should be source available somewhere (whether it is atm or not is still questionable...). Atleast the kernel & GPL'd libs... I'd imagine that certain things (certain closed source drivers under special licenses) couldn't be released, but others HAVE to be...
OSX is Linux based, but it's not an open source OS. Sure, Archos has to release their version of the kernel, but anything else (likely everything else) is at their discretion. Archos has a less-than-excellent history of supporting community development for their devices. In fact, they're pretty famous for charging extra for just about everything you want to add to their PMPs, including codecs.
A quick google search for "Archos homebrew" comes up with naught except a DCEMU forum post about somebody managing to crack the hardware locks on the series 5 players (also linux based) to load a custom firmware. I can find no information on whether the 6 series has similar locks, but I would think that if they didn't, that fact would be newsworthy enough to show up in relevant articles. It does not.
OS X is BSD based, and their license is much more permissive about what you do with their source than most. And yeah, if you do some digging, all of the open source stuff for the Archos gear has source available. The catch is their players will only run signed OS images and binaries, so there's no way to run a non-approved Linux version on their players. Even the Go Fighting Tabby hack was pretty useless in practice.