Not really - all it does is show the networks it can find. You'd need it to show a non-AT&T network as the currently selected network (assuming the phone is from AT&T).
If he does have a unlock code, let him unlock it and show you that it works with a different SIM.
It could be an employee device which are unlocked. If it's truly unlocked, I suggest you test it with a SIM that is not AT&T BEFORE you buy it. If it's truly unlocked, he should have no problem letting you pop in your non at&t SIM. When I sell an unlocked phone, I always let the buyer test it.
The pic definitely does not prove the phone is unlocked as I can do that on my phone, as anyone else can on the their Lumia 920. The ONLY true way to confirm is to test it with a non-AT&T sim card.