Both explanations are correct factors in the overall messy compromise that is tuning. The explanation they gave was attempting to describe string inharmonicity,

Both explanations are correct factors in the overall messy compromise that is tuning. The explanation they gave was attempting to describe string inharmonicity, which is worse for metal strings than more flexible ones and only applies when they are plucked or struck. So violins do not have this problem, because the bowing constantly corrects the frequencies as it slips and grabs. I love your description of the Pythagorean comma, though. The difficult compromise of mathematical purity is what led me to an obsession with pedal steel guitar, since it's one of the only things that can play full chords in all keys with perfect intonation. Edit: apologies for adding yet another mention of string inharmonicity before reading the rest of the replies.