Are you sure? I had read elsewhere that it always goes through the battery, and a sibling comment says there is insufficient power from the charger alone (not really sure how that's possible either though).

Understood. So why hasn't Apple done something about this earlier? IOW, a setting that charges the battery to a maximum of 80% when plugged in?

Because you need to predict whether people want to use it plugged in or on a full charge afterwards.