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What does "easy" mean in this context? From my [ignorant] reading, it sounds like it requires being able compute a fixed point for the hash function in order to be able to integrate it into the program and respond differently under test. I thought that was one of the things cryptographically secure hash functions explicitly made difficult?

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By "easy" I mean straightforward.

Previous examples which showed how instantiating Fiat Shamir leads to an unsound protocol were so contrived that people use to think that they were a testament to how unlikely breaking FS would be [1].

In "How to Prove False Statements", you can actually build what they show.

[1]: e.g. see https://eprint.iacr.org/1998/011.pdf

The attack does not require a fixed point of the hash function to be integrated into the program, it merely involves an implementation of the hash function included in the program, being fed the exact same input as the hash function used as part of the protocol. This is possible because the input is entirely attacker-controlled, so it's easy to duplicate some values as necessary.

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