The meaning of turning all that stuff off is that you should be 100% focused in case of an emergency... it has nothing to do with that it could interfere with aircraft systems...
So something like, oh lets say, a book wouldn't provide any distraction then??
As far as I understand the ostensible reasoning
is to do with interference with avionics.
There was a study done (can't remember who/where though, but I think it was in the US at least) into mobile phones and avionics interference - it showed that there was no effect.
In a light aircraft there is a possible effect when the equipment is so close to the avionics.
In addition if a mobile phone was on at altitude then it would be able to communicate with multiple cells of the network, which wouldn't be likely to work very nicely.
The particular one that puzzles me is not allowing any GPS navigation equipment on a flight. Bizarre...it's not as if they transmit now is it? Although technically if the aerial was close enough to an aircraft's GPS aerial it would interfere.