Even EY's most expensive fares have a sales restriction "TICKETS MUST BE ISSUED ON THE STOCK OF EY "
It isn't blocking sales of cheap infant fares, it is simply blocking sale of its fares to other airlines in general
If EY allows use of YY fares, then I would expect AA to be able to offer a YY fare - if it doesn't , then indeed, will need to be issued on EY stock
Regardless - even if AA was to be able to sell an EY fare, the infant discount would still be the same as if booked from EY