I think that is the case, sorry. On exotic operating systems like OSX
there is a proxy in the driver that serves http through usb (I think) so
you can access the configuration pages without an ethernet cable.
It's much simpler than this, AFAIU. OS X and Windows have drivers for IP-over-USB (compared to the more normal IP-over-Ethernet or IP-over-Wifi). The MOTU box supports this on the other end too, so you're really just connecting entirely normally to the http server on the MOTU, but via IP-over-USB.
There is some evidence of a Linux version of IP-over-USB but from what I could tell it isn't ready for use for this particular purpose.
WARNING: the latest versions of Motu AVB cards seem to break the class
compliant driver. The symptom is that on the receiving end (a Linux
computer running Jack) the channels seem to rotate in blocks of 8. So,
I don't know how "latest" you mean. I bought mine well over a year ago, and it has had this behaviour since I bought it. I was not convinced that it was the MOTU's fault, or rather that it just need some USB audio quirk added to stop it from happening. Alas, the days of me diving into the kernel driver to figure out what is happening are long over.