It's a different usage of the term than the normal case, but it takes advantage of the same vulnerability to length extension. It doesn't necessarily apply to SHA-3 and other sponge functions because there is a difference there between the internal state of the hash function and the actual hash itself. The internal state is much larger than the hash output for a sponge function like SHA-3, which means you can get two messages that have the same hash without having the same internal state, and therefore appending a suffix would mostly likely change that internal state enough to no longer have a hash collision.
> 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).
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.
