I don't agree with that statement. It is a common mistake to say that when the plane is in knife-edge, in a climb or in a dive, the wing is at zero AoA... the AoA is forced by elevator trim and hence is alwais the same, provided that the elevator trim is the same.
In knife-edge flight the wing
must be producing no lift force or the plane would be moving sideways. It would have been more accurate for me to have said the wing is operating at the zero lift angle of attack which for a symmetrical airfoil happens to be zero. If the trim is set for level flight, knife-edge will require some down elevator to hold the line.
Angle of attack is set by trim
and elevator position. Every deflection of the elevator changes angle of attack.