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> It's a different usage of the term than the normal case

Is it? How? It's a simple case of length extension, just that here, since we have two independent starting points sharing the same state, we start with a collision and we extend to a collision.

In other words, these are two length extensions on independent prefixes. It just happens that these prefixes share the same state / hash, hence the surprising result (on a first glance).

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Normally when talking about length extension attacks the original plaintext is unknown, but we can compute the hash of the plaintext plus an extension if we know the hash of the plaintext. In this case we know what the plaintext is, and we happen to have two different texts that produce the same hash, which we can extend to generate many collisions. It's the same property but it's a different scenario than what is commonly referred to as length extension.

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