Can switching step-down regulator loaded with idle LDO consume 6mA @ 6V?
That looks pretty good to me. The datasheet indicates that the chip draws 1.5-2.5 mA when not inhibited and not switching, and if it only draws another 4 mA when switching, I'm actually rather impressed.

The additional current is mostly used to charge and discharge the gates of the two big (5A peak) switching transistors inside the regulator chip. This current ultimately gets dumped to ground; it can't contribute to the output current.