[davidmanze]

Most of smartphone or laptop metal bodies with inbuilt WiFi are made of aluminium and not magnesium alloy. Couldn't find a source which says how good a material shields against HF radiation. But the lighter a material's density, the lower the resistance against radiation, that0s what I thought.

 Not to be downer on your theory but it's wrong! Anything metal/conductive will shield radio waves.

 For example, the whole body of the Pentax K3II is magnesium, Pentax had to do away with the built in flash to house the GPS (under plastic) in it's place in order to receive satellite transmissions, they didn't want change the body design.

 Aluminum foil is sufficient to block radio frequencies (screening) and is often used as such, magnesium has very similar characteristics electrically!