NYC6 produced a reference for his view but it doesn't really say where the opinion it voices comes from. So it is pretty hard to refute.
Iron Man makes a good point which is pretty much where I am but I have a bit more information.
I found this which I think is an interesting read on the n52:
http://www.intlmag.org/files/mg001.pdf
This article says that bmw is using Magnesium alloy AJ62. Unfortunately I cannot find a readily available source for the yield strength. I believe that the aluminum alloy is very likely to be A390 - or some deriviative. It's anneal temperature is unusally high, 925 F and it is made for engine blocks so some thermal heat treatment enhancement of yield strength is credible. Annealed it is about 195 MPa, with the best heat treatment around 300 MPa. There are clearly magnesium alloys this strong but I don't know if AJ62 is.
This discussion of yields strengths is also less useful because the simple way for the engineer to deal with the difference between AJ62 and A390 is to change the cross section. Whichever one is weaker can just have a little more material in the highly stressed cross section to make up for it. In this case the contraction with cooling was also a factor affecting the casting design. Hard to say which was controlling.
I still see no basis for a claim that magnesium is weaker but I have to admit I can't prove it for these two alloys without a reference for AJ62. I hope you enjoy the article I linked, I found it to be an interesting read.
Jim