Given that the most likely emergency is something to do with a train, having a walkway physically separate is an important safety benefit
The emergency walkway runs parallel to the platform at the back of it; albeit at a slightly lower level.
So you'd be the width of the platform further away from -say- a train fire, but that's all.
I'm not questioning the potential benefit of having a second exit from the platform, I'm questioning whether what has actually been built at this location has any real safety benefit; or if it's all about a tick box exercise being complied with at a cost of presumably quite a lot of money.
"Have we provided an emergency exit?" "Yes. Tick." "Does it take you to a safe place?" "Er, don't know, it doesn't require that information on the form."
Luckily none of this takes away the fact that Exeter has a nice new railway station, that I've already found to be useful twice in the three days it has been open. Thank you very much Devon County Council/Network Rail/GWR & even DfT.