Is it true that for a half wave dipole, cancelling out reactance using for example a gamma match does not make the antenna resonant at all, but rather just cancels out the reactance of what is actually a non-resonant antenna which has an impedance when fed from the center of 73 + j43 ohms ?
An strictly half-wave dipole isn't resonant. As you say, it has an impedance around 73+j43 ohms. Since the reactance is non-zero, it is by definition not resonant.
Any manner of matching devices could be added to the antenna, and considered as a whole, they would make a resonant system. But that's not what people usually mean when they say "resonant antenna".
Instead, the antenna can be made just a little shorter. At slightly less than a half-wavelength the reactance drops to zero and the antenna becomes resonant, no matching device required.
In practice this is almost always done, so unless writing in a context where very strict language is required, a "half-wave dipole" really means "a dipole of approximately a half-wave, adjusted to achieve resonance."