Whereas at low speed and α= 18°, there will be an upward force at the taiplane for balance.
It's this statement which points to the part I do not understand.
If I am flying at low speed, and a relatively low angle of attack, with the plane trimmed (and all other things remaining equal), I must apply a pull force to increase the angle of attack (per FAR Part 23.173, and the stick force must be stable through the speed range FAR Part 23.175.
If the plane went from the tail providing a down force to providing an up force, the required "pull" would no longer be required, and there would be a control force reversal, neither of which are certifiable.
If, in a tricycle plane, in this realm of conditions where the tail is said to provide a lifting force, I go from stopped on the runway (on three wheels), to approaching rotation during takeoff, and the tail suddenly provides a lifting force, I'm going to have a heck of a time getting the plane off the ground - or, the tail tiedown ring was sitting on the ground before I started....