Apple will not let Audible install their Audible manager on Macs, insisting that iTunes does the job.
What audible.com actually says is, "Mac users using iTunes do not need to download any additional software. Audible files will download directly into iTunes." To me it reads like Audible didn't even bother writing one since iTunes already downloads their content. But you are right, iTunes does not support the myriad of third party devices out there. However, there is often still some way to get files onto a device. For example, page 49 of the Garmin manual says, "Your device and memory card appear as removable drives in My Computer in Windows and as mounted volumes on Mac computers." If you do some poking around you'll probably find where the audiobooks are stored and can just drag them from the iTunes library to the device. I do this with my non-Apple MP3 player.