"Impedance" as used by audio people is completely different from "impedance" as used by RF people.
In audio, it's about the equivalent resistance of an input or load. If you put a dummy resistor in place of a load, how would the output/driver treat it? How do you choose what value of resistor to use, to stand in for a given load/input? That's the impedance.
In RF, it's about the equivalent resistance of the transmission line itself, because at high enough frequencies the speed-of-light prevents you from even seeing the far end. That phenomenon is well treated here:
I don't believe this is true, AF and RF have significant overlap on the frequency domain, AF is just baseband RF when it's on the wire. RF very much can work with a resistive load, a 50ohm dummy load will ohm out as 50 ohm DC on any cheap multimeter.
The distinction is when the transmission line is "electrically short" relative to the wavelength of the highest harmonic of interest (typical in audio unless your speaker wires are 40km long), and thus its impedance doesn't play much of a role (and therefore you "see" the load directly), or if the transmission line is "electrically long" (typically the case in RF), so your source is driving the line rather than driving the load directly.
In audio, it's about the equivalent resistance of an input or load. If you put a dummy resistor in place of a load, how would the output/driver treat it? How do you choose what value of resistor to use, to stand in for a given load/input? That's the impedance.
In RF, it's about the equivalent resistance of the transmission line itself, because at high enough frequencies the speed-of-light prevents you from even seeing the far end. That phenomenon is well treated here:
https://www.ibiblio.org/kuphaldt/electricCircuits/AC/AC_14.h...