It means Musk & co only paid $31 billion, and only paid tax on $31 billion, while Twitter gets saddled with $13 billion in debt.

$44 billion was required to buy Twitter and pay off existing shareholders. The argument is that under the new ownership the new owners would be able to direct Twitter to take out a loan to further the business, however in practice they avoid tax and saddle the business with debt that it can't afford.