The radio stuff is about "wave impedance", the speaker or headphone stuff is more like a power supply situation at (in RF terms) very low frequencies. You want

The radio stuff is about "wave impedance", the speaker or headphone stuff is more like a power supply situation at (in RF terms) very low frequencies. You want your power supply to have low impedance so it doesn't sag and keeps tight control over its output voltage. There must be a mathematical description of these different situations but I can't be bothered to work it out. An important part of it is probably that an audio amplifier has feedback, an RF amplifier just emits a wave and the consumer must make itself compatible with it. Lower output impedance = more effective feedback control for an amplifier with feedback.