Since the central bank controls the unit of account value and can always devalue and print itself out of any sticky situation, the only meaningful way it can go "bankrupt" is to lose the ability to appreciate the currency. I agree it's a stretch.
The notion that the central bank can conjure pure profits out of thin air and has no liability against its issued currency is fantastical, it would suggest a perpetual profit machine. In reality, any issued currency comes with a strong liability to buy it back in inflationary times, so it's clearly an external debt from the CB's perspective, this is a well established notion.
This has origin in the pre-modern, free banking era, where competing private banks issued "bank notes" attesting their demand deposits, and which in time became widely accepted. This is basically the exact thing a central bank does, with a state granted monopoly, and no link to the gold and silver the bank "owes" for the notes.
