I've looked it up and know that "that's that" has several meanings in different contexts. such as "a piece of cake" or "it's done(reach a conclusion)". I can't decide which meaning it is in this context, I think both are OK.
We decided the fairest way to settle this argument(whether 1 is a prime number)was to do a survey of 100 million Murderous Maths readers. Here are the results:
is 1 a prime Number? Votes
don't know 211
don't care 99999774
The Murderous Maths Organization can now proudly declare that the number 1 is NOT prime by 8 votes to 7.
So that's that. //