> It's not principled attribution, just locating the most similar example in the training set.

This is a profoundly important distinction.

Back-tracing data to contributory training examples is a genuine "influenced by" relation. Picking the nearest neighbour to a given result (even if its an exact copy!) cannot say anything useful with respect to origins. And given that there will always be some proximate neighbour, it's really a "misattribution machine".

This is bit like how our broken patent system grants or denies ownership of a design based on similarity to extant art but regardless of actual originality.