the heart of part 2 was that we could no longer work with Ints, but would need to work with longs

I only partially agree. If you inspect the input you see that one monkey has the square operation (old*old) , after 1000 rounds it will be computed many times.

knowing how large a 64 unsigned bit integer could be, should be clear in a short time that 1000 rounds are going to break that, because every square operation adds so many digits as the magnitude of the number squared.

So it is not exactly black magic.

I had exactly the same issue. I realized/guessed it was 'another operation' but why I should multiply the random prime numbers was just not a suggestion I'd come up with. I had many other ideas that I tried that produced good but incorrect results...

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There are multiple solutions. They're both pretty much the same thing and can be mathematically shown to be equivalent, but there still are multiple solutions.

There's already complaints about people thinking that that line means you're supposed to find an operation that like, magically matches the sample input, or something of that sort, which obviously isn't the case but your suggested rewording would lead even farther in that direction.