The flats on the motor shaft are probably just bearing surfaces for set screws. I've seen bores like this, but only in punched sheet metal parts where fit is not an issue. Actually, as drawn, this hole can not be machined, only punched or broached. If you want to go into mass production, punching 5mm alum is not a problem.
Would it be too obvious to just bore a 10mm hole and then put two set screws in? This pulley doesn't look like it will be under too much torque load, so two (even one) set screws would be more than adequate. If you're into heavy torques, the shaft would bend first.
Bruce