i agree, but from my understanding, the most important factors in an oil system is pressure and flow. In order to meet these two, there must be a sufficient level for the pump to function regardless of what the sight glass shows!
I perceive the sight glass to be a physical check incase the computer is showing either low level or low pressure. The level gauge lines are set to provide the pump with enough oil during all running conditions! i.e. round the world, on off road etc.
However, in the manual it does say that if level is low from the sight glass, top up at the next fuel stop. But says if the pressure light comes on, stop immediately!
For the OP, i think it says in the manual if there is oil in the sight glass, checked in the way the manual says, and the oil pressure light is on, seek BMW advice.
If the oil light is on due to low pressure you'll hear the engine noise before you've stopped and called the dealer.