Alright, so take these results with a massive pile of salt. I have no idea if this is a valid measurement, but it's the best I can do with the materials on hand.
Instead of a resistor, I used something with a known impedance: My DT 770 250 Ohms. I plugged a double-sided 3.5mm male into my S10 and measured the voltage of a
60 Hz sine wave until it held a consistent 0.5436V.
Next, I put the extension cable's plug and the DT 770's plug side by side and laid the multimeter probes across each of them so they're contacting both. I kept measuring until it held a consistent 0.5398V.
Using these two voltages, the output impedance of my S10 (SD855, not Exynos) can be calculated to be 1.76 Ohms.
Unfortunately, I don't have an S10e.