In my days (and that dates me) it was 1800 Hz three-wire synchro, and the EMI was from the 400 Hz, not from 800 MHz
EMI emissions covers the whole electromagnetic spectrum there buddy! From 1Hz up to 60GHz or higher!!! In addition, a device may operate in a certain frequency range but there is a phenomenon called harmonics. Harmonics are energy levels at multiples of your fundamental (Operating) frequency. Most electronic devices today emit some form of electromagnetic energy in the form of either conducted or radiated emissions or both. Radiated emissions is our suspect here and is a headache in todays society for wireless technology.
I suppose we're now talking multiple digital data buses? If so where and how are the data put on the bus and where are they taken off?...What baffles me is that a laptop or other RFI-producing device in the cabin could even be thought to
a) interfere with a shielded bus,
b) introduce enough similar interference on all four buses (I presume there are four) for it to be interpreted as a valid command.
I'm not too familiar with the architecture of a FBW system but I suspect that inputs from pilots and/or autopilot are being converted from the analog domain to digtial domain (ease of signal processing) which is then processed in a computer that filters input signals and then converts the information from digital domain to the analog domain for the hydraulic actuation of the control surfaces. In addition there, is usually some kind of feedback response to the computer for stability.
There are tons of ways for EMI to interfere with the FBW system. In the world of RF (Radio Frrequency), energy can creep in where terminations occur on data lines. I've seen it! There could be an issue with your data lines termination which may cause mismatching, there could also be a noise immunity issue on the front end of the computer (transceiver). I could go on and on...But, I leave the speculations to the investigators and researchers.