Flat Back

Beacuse they are mistaken.

cheers mate
this thread can be closed down now.

Fabian agrees. The thread can be closed.

Bottom line is that in a windtunnel, a flat back might be a bit more aero however the real issue is in what aero position the rider puts out the most power.

I misread the title, and now I crave bacon.

Actually, it's what position produces the best combination of power and aerodynamics.  And, for most of us, comfort fits in there as well.  For the pro cycling TTers, comfort is less a factor.

i was looking for speed
i get comfort from sitting on my couch

where you looking at my fit thread this morning....

at least you know that some advice given there was not correct

It's not that your back is rounded.  It's that it appears you are rather 'upright', i.e., not low in front, and catching a lot of extra air with your body.  That's principally what PennState referred to when he said "un-aggressive".

because some people assume that flat back = more aero and that is not always the case (think of Lance and his hump like back - I happend to have the a similarshape, though  slightly less power. Yes, just a tad! )

What matters is to reduce your frontal area and in general if your position is lower in the front (hence the flatter back) frontal area can be reduce, but unless this is tested (wind tunnel, with power meter, video, etc) you can't be certain. Some people in their quest to get lower and flat their backs may be lower in general but too low to a point they can't comfortably hold a good aero position hence while you might be lower height wise, you might be effectively increasing your frontal area by been wider. The other thing to consider as JK said is power vs aerodynamics aka as watts/CdA. You want to find the lowest possible CdA while been able to generate your usual power so you get the best ratio out of the two.

Plus no matter how aerodynamic your position is if you can holded it for your racing distance and you end up sitting up like a sail into the air!