That firmware linked in the first post does have a normal USB handler procedure inside. But this doesn't imply USB would work, of course.
So how are you going to use that hidden USB? Even if you would be able to connect and use Garmin USB protocol, that firmware doesn't seem to support command 0x60 which dumps regions.
And if you're not going to use Garmin USB protocol, then what are your plans? Are you trying to reach JTAG?