So to circle back on this, Edward was able to get this resolved actually with the help of another OAK user. I tried to help over email but was not much help relative to the other user. The issue is particular to macOS 10.13.
The solution is:
brew install libusb
python3 -m pip install depthai
step 3: then trash the just installed
step 4: create a symlink in its place to the
libusb-1.0.0.dylib created in the first step.
It appears that the install script in step 2 is the real cause of this problem; that is, it is installing a 10.15 version of libusb-1.0.0.dylib. So this is why deleting what it installs, and using what brew installs from the first step is what fixes it.
(And thanks EdW for having circled back with the solution.)