You're referring to an article that was pretty dated when it was written four years ago. And it was written in reference to titanium frames, not aluminum.
These days aluminum tubes are hydroformed, which means they have various cross sections, they're fatter on the outside at the ends where stiffness and strength are needed, and thinner where flexibility or saving weight is needed. And the butting on the inside is totally variable. Hydroformed tubes on mountain bikes was becoming pretty widespread starting 10-15 years ago.
So unless you're shopping for a vintage mountain bike, you should be looking at the overall ride characteristics and purpose (XC, trail, all-mountain, freeride, etc.) of the frame, not the shape of the tubes.
