Yeah, I remember reading something like that, but isn't the United app required to pay for the meal on the flight, so that they know which credit card to charge? To be honest, I have never been able to understand their payment process, and since the United app doesn't run on my phone, I just gave up on the in-flight meals.

The discoverability is horrible, but in this case there actually is a somewhat accessible way to pay: if you visit https://united.com/inflightpayment, you can add your card details for a specific flight before the flight (possibly also during the flight using in-flight WiFi, but don't quote me on that).

I actually find it more convenient than presenting a card in person, because my country uses chip-and-pin cards and many American (soft-)POS terminals break when presented with one. There's also less of a chance of my bank arbitrarily flagging the transaction as suspicious.