They were testing next years NON coanda exhaust and both drivers said "woo, the car's better" so it turns out their coanda exhaust was likely just always ****.
Which makes no sense, as there is zero relationship between the current engine setup and next years exhaust layout. Next year is a single exit, on the cars centre line that exits at the rear of the car. Current exhausts are twin exits on the sides.
I assume they were just using a neutral exhaust to evaluate other aspects of the rear end without any blowing affect.