How does Apple achieve the all aluminium enclosure with RF on the MacBook Pro?
Apple cleverly places the WiFi antenna assembly adjacent to the cooling vent to seemingly "circumvent the laws of physics" and get around the Faraday cage problem.

Looking at the underside of the MacBook Pro (15" 2017), you can see a cooling vent that goes for about 80% of the width of the machine.





The WiFi antenna assemblies are put on either side of the MacBook directly adjacent to the cooling vent.  So, while it appears that the WiFi antenna is hidden behind an all aluminum case, it's actually out in the open which is how it receives WiFi signals in closed-clamshell mode.