I drilled my old Tubus rack slightly to fit M6, but the Old Man Mountain one is a bit different. Am M6 bolt fits through, but the lower mounting point uses an additional spacer that is designed to increase the strength of the connection. Drilling it out would leave the metal of the spacer too thin, and I can't use an M6 spacer with a wider outside diameter as it wouldn't fit into the recess on the rack.
Luckily the thru axle system is really good. The upper mounts accepted M6 with no drilling and the whole process was really easy - although they don't offer a nice diagram for the order of nut, spacer, split spacer, bolt line Tubus has for all of their racks