the increase in output for a given input as the amp heats up is due to increased conduction in the transistors,
as the transistors heat up they conduct harder, operating point moves up towards AB not down towards class C,
if the bias voltage was dropping more than is needed to counteract the increased conduction of the transistors output would drop not increase,
a thermal tracked bias supply is used to counteract the rising conduction of the transistors with rising temperature to maintain a constant operating point/stop thermal runaway,
if the bias supply source impedance is not sufficiently low the bias voltage will sag with increased drive level causing a drop in operating point towards class C, your modulation will modulate the bias supply,
class C having better efficiency than AB is only true when both amps are designed with correct load lines for the supply voltage and operating point they are using,
when used on stock voltage, class C amps of the davemade style draw more current per watt of output than any properly design class AB amp i ever came across because they are designed with load lines for running elevated voltage in keydowns,
the often touted class C efficiency advantage in this example is a fairytale, they have very poor efficiency when used on stock voltage,
it gets even worse if the clone builder throws a resistive divider + untracked diode bias circuit in there to keep the ssb crowd happy,
another one for cb mythbusters.
