> > they make things that are near each other in their domain (inputs) map to things that are far apart in their codomain (outputs).
This is describing a specific subset of hash only. *Cryptographic* hash functions map inputs to outputs with high and uniform dispersion.
So you are talking about cryptographic hashes, but different hash functions can have different properties.
ThumbHash is absolutely a hash function, which is "any function that can be used to map data of arbitrary size to fixed-size values, though there are some hash functions that support variable length output." (https://en.wikipedia.org/wiki/Hash_function)
