You have misunderstood or mis-read the article ... 2^{31}-1 is not 12345678910987654321.

Specifically, 2^{31}-1 = 2147483647.

Borel asked Dyson to name a prime number and, unlike Grothendieck, Dyson provided a number that is only divisible by 1 and itself: 2^{31) – 1.

But that reply did not satisfy Borel. He wanted Dyson to recite all of the digits of a large prime number.

Dyson fell silent, so after a moment, Sloane jumped in and said, “1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1.”

So Sloane was supplying a different prime, but one where he could recite all the digits.