Gotta love left side logic, that was very clever of him
As for the why not do it for 1000 - cos the pattern has already been derived and just a waste of time after this just finding the additional primes .. think they are 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97