If the thrust angle is off the plane will tend to change pitch with changes in throttle. Too far down, and the plane will lift its nose when you let off and drop its nose when you apply power. Too far up, and the opposite happens. When the thrust angle is just right, the planes pitch sensitivity to speed is minimized. Proper thrust angle changes with changes in CG and wing/tail incidence also, so those need to be set right before tuning the thrust angle.

If you decide to build the plane with less down-thrust the main risk is it might make the plane want to gain altitude when you advance the throttle after trimming for level flight. Can these two quotes be reconciled or am I comparing apples and oranges? What happens to one AOA one speed?