Hello Ken,
That's how I define it.
Not really. In an RF amp such protection schemes are typically much slower than the slew rate of the amp, usually by orders of magnitude. That is one reason why, for example, an over-voltage condition after an antenna cable break blows the final transistors. The protection circuit isn't fast enough to curb power.
It is possible to design true feedback that sets the impedance as you describe but I have rarely seen that in RF gear. Usually there is just a fwd-ref bridge plus a current shunt and those signal to a uC or (slow) discrete parts.
Not the short term "true" output impedance. In this quite typical case the bridge and shunt signals went into multipliers (fast), then into a micro controller (really slow compared to the RF). Only after the uC had executed its respective code would the protection scheme do something. Probably in the order of milliseconds.
Actually, the shunt signal also had an override path, curbing power while bypassing the uC (still slower than the RF stage though). But this only means that the output impedance was curbed to a lower limit for the high power range. When operated at, say, 20% power it wouldn't do anything to prevent you from connecting that 30ohms load versus 50ohms nominal.