This is true. A bottle on the downtube or no bottle at all hardly makes a difference. It's one of those wonderfully inexplicable aero results, but it happens all the time. We did it once again with Frank Schleck in the tunnel, riding with a bottle on the downtube, then tossing it to the side and the drag hardly changes. It does however seem to be important that the bottle is not too close to the rider's feet.
But put one on the seattube and all hell breaks loose. Not a problem in training of course, but in racing keep it on the downtube only, or use a different hydration system altogether (ADS, behind the seat, Camelbak, etc).
Gerard Vroomen
3T.bike
OPEN cycle