Yes, kNN is the bound if you formulate learning as: there exists an f_true: Pixels->Animals; we have an infinite sampling of (Pixels, Animals); kNN performs at 100%. Therefore kNN is the bound.
Whereas I dont think this has much to do with learning. Learning for me is: the function probably doesnt exist, we dont know what its domain would even be, and we have no idea how to sample that space. You know: the history of human civilization, but also: the history of a day of any animal's life.
We encounter problems daily whose functions can only be modelled by partial input and output domains, and our job is to find ways of resolving the inherent ambiguities in those situations. Ie., to find what parts of the world are relevant, to find ways of measuring it, and hence to find more data until the ambiguity resolves itself.
kNN being a perfect algorithm is a catastrophe for this whole formulation of AI. All it means is that the whole field of machine learning has as its limit what you can do if you already know the answer. You can see from this that it isnt learning at all.
The situation is exactly backwards: ML assumes relevant well-carved unambiguous data. The whole problem of learning is precisely to produce it!