Quite often this kind of instruments might be "tradable debt instruments" which can't be converted to actual bitcoins, but can be traded back and forth on the platform to traditional currencies. The issuing company maintains the peg to the bitcoin market value, probably using futures or similar instruments.
Of course they might also enable bitcoin withdrawals/deposits but I somewhat doubt it. There are similar offerings on other traditional platforms and usually they work this way.
Of course they might also enable bitcoin withdrawals/deposits but I somewhat doubt it. There are similar offerings on other traditional platforms and usually they work this way.