The bottom board includes an exposed lip which extends from the front of the hive. It is exposed to rain and functions as a landing area for the hive. If it tilts upward (the rear of the hive, then is lower) then rain tends to flow into the hive, pooling on the bottom board. Therefore, you want the front landing area of the hive to be lower so any water drains out. Condensation, rain, whatever.
However, the Flow Frame guide suggests that the hive be tilted down, toward the rear, so that honey flow is downhill flowing out the hive rear, the backside.This requires a change in angle when draining the flow frames -tilting forward normally, reversing and tilting backwards when draining honey.
After thinking about this ----- two thoughts---- First, it does not appear in the videos that anyone using the new flow frame changes the hive tilt. They look level. Second, if tilting is required it would be easy enough to place a small dowel or triangular wood strip under the middle of the hive with a removable wedge under the rear of the hive. The wedge is removed and the hive tilts back when removing honey. Just add a brick or some weight to the hive roof, on the rear and the hive tilts.