The reality is that any spline mounting system will have the potential for backlash. It requires an unnecesary amount of precision to prevent it. If you consider the application you'll see that the backlash isn't an issue because bicycles don't have a reverse. So the first time you apply the brake, the wheel will roll forward against the disc taking up any play and will stay that way forever. The backlash would only become a problem if you applied the brake and forced the bike backward against it.

That said, you do want enough side pressure on the disc ro keep is secure against small reverse torque loads, and if you find it moving, should consider either tightening the lockring, or adding a spacer between it and the disc, or using traction material between the disc and hub face. When you do that, partly tighten the lockring, roll the wheel against the braked disc to take up the backlash, then tighten fully.