It’s a simple math trick, the kind that “human calculator” types use to impress people. Any base number that you can reduce to 1 by this method (adding the digits), when you multiply it, the result will reduce to whatever number you multiplied it by. You could substitute just about any number for the number he is using for a “millenium”, as long as it reduced to one, and it would have the same “mathematical seal” that he is talking about here.
So, as you can surmise, a “seal” that can confirm an infinite number of solutions as the correct one is mathematically worthless.
And this also, improbably, attributes the theory to the prophet Daniel. While Daniel prophesied in mysterious numeric puzzles, even he didn’t get anywhere close to this wild.