Output impedance is not only the amplifier’s. Everything between the socket and the driver adds series resistance: the adapter, the extension lead, the thin stock cable. It all lands in the same place in the arithmetic, and on a low-impedance in-ear it can matter more than the amplifier did. This adds it up and tells you what it costs you.
Copper resistance per gauge is a standard table, not a manufacturer claim: a 24 AWG conductor is about 84 mΩ per metre at room temperature, and each step up in AWG number is roughly a factor of 1.26. The signal goes out and comes back, so a cable of a given length contributes twice its one-way resistance — the single most commonly forgotten factor of two in this hobby.
Two things this deliberately does not do. It does not model skin effect, dielectric behaviour or any of the other mechanisms cable marketing invokes: at audio frequencies and these lengths they are orders of magnitude below the plain series resistance, which is the only term worth arithmetic. And it does not claim your cable sounds different — it tells you how much series resistance you have added, and lets the one-eighth rule say whether that is enough to matter.
On a 300 Ω headphone you can add a couple of ohms and stay well inside the rule. On a 16 Ω in-ear the whole budget is 2 Ω, and a cheap adapter plus a long thin lead can eat it on its own — before the amplifier has contributed anything. That is the asymmetry worth remembering: the people who worry most about cables usually own the headphones where it matters least. The full one-eighth explainer, and what the tilt actually looks like.