Earlier quoted context omitted.
Reinforcing fins can do quite a bit. Macbooks don't have a lot of flex. However, Apple has some patents that cover building up solid aluminum with stir welding instead of CNCing out of a solid block. You might have to challenge that patent (prior art, Boeing patents that may be older) to make it cheap, but if it's enthusiast targeted, an extra 50 bucks or whatever might be just fine for people.
You cannot weld aluminium, or at least not on the cheap. The only real "weldable aluminium" known today are scandium-aluminium alloys which are both hard to get, and expensive.
You have probably flown in an airplane with stir-welded aluminum. Stir welding is a bit like... making velcro and sticking it together at exactly the same time.
Nobody is building things out of 'aluminum' so it feels like nitpicking to argue about metalurgy for a company that is just going to buy stock in block or plate form. With a bunch of electronics nerds.
I only brought the metal in to talk about stiffening and because some patents may make it difficult for them to build a work-alike that lives up to their hype.