He says that pot opt refers to the future only however I have come across many ex. where it is better explained as referring to the present. like Г40 καί κε το βουλοιμην καί κεν πολύ κερδιον ηεν.
You may find it difficult to think of the potential optative as referring to the future but if you think about it you’ll see that strictly speaking it does. E.g. ειποιμι αν “I would say” (sc. if you were to ask me), etc. etc. So too here with καί κε το βουλοιμην, “I would want it,” i.e. (If you were to ask me what I’d want,) I’d want that you had never been born. Then καί κεν πολύ κερδιον ηεν (not potential), “and it would have been much better” (if you never had been.)