The short answer is that they welded those massive aluminum joints with DCEN and Helium shield gas.
There is some technique involved, and surface prep becomes even more important and regimented. (others here have more experience so I'll leave it to them)
But it is a pretty common process in the Aerospace world for joining thick sections of aluminum.
It is possible, but like Lawrence mentioned it is typically done on thicker material and surface prep is very important. Since there is no AC cleaning action all oxide need to be removed. Welding aluminum on DC- is a bit tricky at first, but once you get a feel for it, it isn't too bad. The biggest problem in my experience was just knowing when the puddle was ready to start adding filler. Since there is no AC, the puddle doesn't give the nice shiny appearance to let you know when to start adding filler. After having the puddle fall out on me a few times I started to get the hang of it. When I had to test to qualify for my certs, we used an argon/helium blend in a gas mixer. It's been too long now and I don't remember the mix percentage.