when the US central bank (government owned)
and the US treasury(government owned) have a debt between themselves
.. is it truly a debt
Well, it may look silly at first, but when you consider the (smaller) debt that US owes to other countries, any kind of movement in the big debt will send reverberations to all the other national debts. It's not like they can just write off the $31T and pretend that it didn't happen, because other non-government entities have other loans with the central bank and the Federal reserve.
They can't just act as if it doesn't exists, but the problem they have right now is within and not with other nations. It's basically US government saying that you can't be in this much debt to itself, and blocking to get even more debt, and that causes a bit of a trouble, because in order to grow you need to make a debt.
Imagine yourself like a factory owner, you make 10k breads a day, and make 10k dollars in profit, if you take a loan of 1 million dollars, you can buy machines that could make 20k breads, and make you 20k dollars per day, and you can pay the loan back easily, but now you have 1 million dollar in debt, next day, you want to get 10 million in debt and make 100k breads and 100k profit a day, the next day you want 1 billion in .. you see? Other nations are fine with it, because they know USA can pay it back, it's USA within that's arguing it.