How much effect do everyday EMF fields have on cell phone hardware?
What these researchers seemed to be doing was accessing the data from within the device using an application. External EMF could absolutely induce spurious noise and change the signal. The thing is, for these devices to work, they have to be fairly immune to noise.

Also, their technique would likely involve a series of filters. Once they had the raw data, they would have to take it apart and look for the "fingerprint." That would have to exclude noise and anything common to all devices.