Why is aerodynamics important on mixed-surface (asphalt, gravel, fire roads, single track) rides?
You may go slow off-road, but on the connecting asphalt sections you need road riding speed.
If you’re going slow because of gradients, aero indeed doesn’t matter. But often you go slow due to head- or crosswinds, so you're slow relative to the ground but very fast relative to the air and that’s what counts in aerodynamics.
The key to the Exploro’s speed is the aero downtube that is wide enough to catch the airflow coming off the big front tire (a skinny downtube wouldn’t even touch the airflow). But at 50mm wide, we cannot use a full airfoil shape as the tube would be 150-200mm deep, causing too much surface drag. So we square off the tail, keeping most of the aero performance with a 50x75mm cross section that is also close to perfect for strength and stiffness; our Sqaero shape. The wide downtube also effectively shields the water bottles.
More details on the 3T Exploro LTD frameset.
