καί μου τότε οὐκ ἐμάνθανες.
why G? this verb takes Acc and there is no noun on which G may depend.
My read: You weren’t learning [it] off me. [from me]
The actual object of the verb is what’s being learned, but the genitive shows the origin. It works like ἀκούω. At least that’s how it strikes me.
yes, this is ablatival G I have found in Sm an example with manthano