The imaged guide is meant for a brake cable, in addition to the two gear ones. The third tunnel is centered between the gear guides as a under the chain stay brake has it's cable usually running to the center of the caliper/arms (as in a roller cam or u brake). Whether said guide will locate the hub brake cable's path well and not contact the tire remains to be found out.

It's not hard to come up with alternative cable routing devices. Short lengths of casings zip tied to the DT and stays is but one simple option. Andy.