Our 148mm boost 2020 rear hubs utilize a new axle system.
The non-driveside endcap is captured on the end of the axle by 4 threads. These threads are not used to preload the axle, but simply prevent the axle assembly from coming apart when off the bike - a trait that plagues some other hub manufactures. The captured endcap system keeps everything together.
A small amount (approx 2-3 mm ) of side-to-side play is present when the wheel is off the bike, but should not be present once the wheel is installed on the bike and the rear thru-axle tightened down.