Question about steel tube shapes.

Not exactly a repair question per se but figured this crowd could answer it.

I have a 2012 Opus Legato. It's made of triple-butted crmo but the downtube is shaped like an AL bike - much bigger than the top tube.

According to this Ibis article increasing the diameter of a steel tube increases stiffness but also increases the risk of "beer-canning" where the tube is at greater risk of buckling. Considering this is triple-butted steel, i.e. thin walls in a buckle-prone zone, it seems like curious design.

Is there any advantage to a fatter downtube as opposed to the narrower tube diameters seen on most steel bikes? If it's not into the buckle-zone ratio stated in the article of 60/70-1, why aren't more steel bikes built like this?