Perhaps they do not need meals to do Mint Jr. , just because that is traditionally done does not mean it always has to be that way.
F meals on AA/DL/UA are not great in any case
It's still possible to do decent meals such as gourmet sandwiches and salads even without ovens. After 911, United was only doing cold "deli plates" in United First and United Business on domestic flights of up to 5 hours and they were actually very tasty.