Saying Al is stronger than Mg is somewhat subjective and doesn't relay all of the necessary info. For one thing, engine blocks are not forged, but cast (the article notes forged Al is stronger...).
Engine blocks are made from a cast Al alloy or a Mg alloy (or both in BMW's case). (Mg is a very common alloy element in Al alloys by the way). Now what neither of you know is the ALLOY that BMW used to make their block, so you cannot compare the two. Also, overall strength is determined by wall thickness, so a lower tensile strength would just require a thicker wall design to compensate. The real overall design question should be, for the given alloy strength and necessary wall thickness, which block design would be lighter (or better meets the overall engine design criteria, don't forget cost as well).
Arguing which one is better is like pissing in the wind. It all depends on the overall design of engine, application and powertrain.