I have something similar on mine, not as bad as that, but a few spots here and there, Nissan dealer agreed with me that it's under warranty and has put the claim in motion. It'll take a long long time though, Nissan HQ are in no hurry to pay out. But I think there are a few members on here who have claimed and got new bonnets because of similar corrosion.
I would be getting back at them and asking them to define what exactly warrants a claim. Are they suggesting the car can corrode and shed all it's paint, but if there is no holes, it's ok?
What you and I and many members have is the earlier stages or corrosion before it eats right through the metal, it's only a matter of time. Besides it doesn't cost the dealer anything, HQ supplies the replacement parts and should be compensating the dealer for any work undertaken.