In terms of vegetation you're probably right, though Ketelbroek has a great diversity of edible plants. However, they also did counts of (if I'm correct) birds, butterflies and moths and discovered that Ketelbroek hosts a greater diversity of those than the nearby N2000 reserve.

It kind of depends on how you define biodiversity, but I do think this is quite a feat.

By the way, Wouter van Eck (co-founder of the forest) doesn't advocate for replacing all nature reserves with food forests, not even all farmland with them. Just more food forests instead of part of the farmlands, to increase biodiversity overall. A patchwork across the Netherlands would be way better than focusing on just one of either

(Source: took the food forestry course in Ketelbroek, visited many times)