Why would a camera maker promote an aluminum body over magnesium alloy?

They are touting that the parts are CNC milled from billet aluminum compared to cast magnesium as is commonly used. Both are alloys and probably contain some of the other in their mix.

In general, if you compared identical parts; one milled from billet and one cast from the same material, the billet part will be stronger. But that is not the comparison. Magnesium alloys are both lighter and stronger, particularly in thin wall construction. It is also less prone to corrosion.

I agree with Michael... it's probably just hype. In this case, besides being a somewhat cheaper material, it also has much lower initial production costs. Programming a CNC mill is a lot quicker and cheaper than creating high precision molds. There is a lot less risk compared to machining magnesium (the cast parts are still machined/finished). And it's also a lot cheaper to change/abandon in two years when it's time for the next "upgrade."

TBH, I think it was a smart move. But no real benefit to the end user (unless some cost savings are passed along?).

FWIW, the Sigma fp is being milled from billet magnesium...