they’re sourcing whatever RAM they can find, so some units ship with 1x16GB and others ship with 2X8GB and it’s just random luck which you’ll get

In a later statement from VALVE they cleared up that this i not what they meant to say. All Steam Machines come with 1x16gb at the moment. But it might change in the future and then they cannot or don’t want to guarantee either setups. There are no Steam Machines with 2x8gb from VALVE at this moment.