It was probably total coincidence. If you give a dog a command they've never heard and they aren't sure what to do, they'll often try and do something they've already learnt. It might be total chance that he chose the laying down action and then figured thats what he was meant to do being as you didn't correct him in anyway.

There's no way he could have known any other way. Even humans couldn't do that so I very much doubt that dogs can.