Learn in the sense of "teach" dates from the 13th century and was standard until at least the early 19th.
made them drunk with true Hollands—and then learned them the art of making bargains — Washington Irving
But by Mark Twain's time it was receding to a speech form associated chiefly with the less educated.
never done nothing for three months but set in his back yard and learn that frog to jump — Mark Twain
The present-day status of learn has not risen. This use persists in speech, but in writing it appears mainly in the representation of such speech or its deliberate imitation for effect.
Some languages use the range verb for learning and teaching. People that have those languages as primary language will often use learn in place of teach when producing English.
The strange thing is, it's vaguely possible that "learn" in the sense of "teach" might have been hanging around in dialects of English since the similarly sounding word "leren" was still in use. "Leren" sounds an awful lot like "learn" but is completely unrelated and actually does mean "(to) teach".
Modern Dutch still uses "leren" this way. It's even spelled the same which is kind of rare.