The problem is, at 1E+15, XL start dropping significant digits. It can't display this:

1,000,000,000,000,001

In your linked article, this is shown under the Number Precision category.

Since it no longer is retaining the digits from the ones value, all numbers become even, and no more primes can be found. The largest prime that XL can find is:

999,999,999,999,989