Since I'm a cyclist and not an aerospace engineer, I don’t have access to a wind tunnel to measure these things. What I can say is that my bike performs perfectly in all situations, even on descents at 80 km/h. I've never noticed any weird noises or whistles like the ones you get from roof racks on cars, so as far as I’m concerned, the square downtube isn’t a real issue at all.
The drag would have to be insane for you to be able to hear it.
A bus isn't aero. Therefore, that downtube can't be aero. Would you be faster on a kamm tail shaped down tube? almost certainly. Would you EVER see a bus shaped downtube on a TT bike? No. Do you see kamm tail shaped downtubes on TT bikes? Pretty much 100% of the time.
Now, does it matter?
If you like your bike, great.
From my perspective, most of my riding is done on the flat, so i do care if i can save 5w at 40kmh, because i often ride at 40kmh.
And from the perspective of people considering the frame, they should know that this feature can't possibly be fast. Whether it matters to them or not, is for them to decide.
My frame arrived today and all my worries about quality issues faded 
Frame is a top notch without any wringles, cheese and etc.
Lovely bike!
