The reason is simple: individual holders of Japanese government bond may want to get out of Japanese government debt. These individuals then hold Yen in a bank account (if they're regular individuals or institution) or in a central bank account (if they're banks). What happens with those Yen?
Maybe they'll buy some other bonds, or sell those Yen for another currency, or something else entirely. But no matter what they do, those Yen will still be around. It is impossible for them to disappear, unless somebody buys Japanese government bonds.
So those Yen might circle around a bit in the financial system, but at some point, they will end up with somebody who sold some asset and does not want to buy anything else. This somebody now has a choice of keeping Yen (which guarantee no loss of principal but have zero nominal return) or of buying government bonds (which also guarantee no loss of principal and which have a - however small - positive nominal return).
At this point, buying the government bonds is clearly the superior option. This hot potato effect of money is why there will never be the need to print gazillions of Yen to get rid of the debt.
I would point out that one might say that your statement is based on a fundamental misunderstanding anyway: Whether you hold Japanese government bonds or Yen, both are forms of government debt! The only difference between them is in maturity and interest rates.
The literal sense of the word "literally"?
Yes, I think that's exactly what was meant!
Has the word "literal" been so badly abused across the internet (true) that you thought a correct use of the word needed clarification?
Or is contemplating the fact that Japan's govt. debts are entirely in its own currency, which can be issued at will with no constraint from the financial markets, so shocking and nonsensical to orthodox thinking that you thought that the original statement couldn't possibly have meant what it said?