[imoverclocked]

We can further caveat that it wasn't "anywhere that had a 5G service" since the radio altimeter is not needed except on low visibility approaches into an airport.

Either way, it's about interference with aircraft systems from cellular infrastructure/devices. Certainly, if a tower talking on 5G can interfere with a radio altimeter, a bunch of cellphones onboard could do the same thing or even amplify the effect.

> it might become a Federal offence not to have your phone in airplane mode, for example - but obviously it’s not

Violating directions from the flight crew can be a federal offense. There is a chain of authority from the FAA to the airlines and to the PIC. The airline and PIC delegate part of that authority to the rest of the crew. Unless you are very aware of every regulation and particular company policy, I highly recommend that you don't test this.