Thanks for the tl;dr. However, the fun fact is not true for surjective functions, IIRC, in which case multiple inputs may relate to one output, if this is relevant for WaveNets.
Nitpicking: surjective functions do not relate to unicity of ouptuts; you'd rather talk about non-injective functions. I agree with your point, though.
(surjective != non-injective, in the same way that non-increasing != decreasing)
(surjective != non-injective, in the same way that non-increasing != decreasing)