Great solution!

Return statement might been more readable without the ternary operator.

Like

return new ulong[]{fib1,fib2, Convert.ToUInt64(fib1 * fib2 == prod)}

or even

bool found = fib1 * fib2 == prod;
return new ulong[]{fib1,fib2, Convert.ToUInt64(found)}

I made same misstake, e.g.missed the specification

"a valid response word must contain four different letters"

tent contains two t and is therefore an invalid answer.

I made same misstake :)

e.g.missed the specification

"a valid response word must contain four different letters"

nett contains two t and is therefore an invalid answer.