IFE systems do show telemetry data from aircrafts' EICAS, though -- things like ground speed, altitude, temperature, etc.
I'm not sure on the specifics since Boeing has kept silent about the whole debacle, but I do wonder whether the IFE and avionics are airgapped on all aircraft, and whether you could exploit them if they aren't.
It's that very silence that makes me suspicious. Security through obscurity is only a reasonable defence if you know you are otherwise insecure. That said, it's also common when big companies have security concerns, valid or not.
I'm not sure on the specifics since Boeing has kept silent about the whole debacle, but I do wonder whether the IFE and avionics are airgapped on all aircraft, and whether you could exploit them if they aren't.