It entirely depends on what is done with the coin. Initially when the coin is minted, yes, they are still owed money. Nothing has changed for them. Eventually

It entirely depends on what is done with the coin. Initially when the coin is minted, yes, they are still owed money. Nothing has changed for them. Eventually, the treasury bonds that they hold will expire and the government will pay them back. At this point, they would usually just buy new treasury bonds. However, if the government stops issuing new treasury bonds after minting the coin, then they will no longer have that option. They could buy other (foreign government or corporate) bonds, but the money they use to do so just changes hands. It ultimately stays in the system at some bank account, which ultimately translates to high-powered money held by a commercial bank in a Fed account, where it only earns interest as long as the Fed pays interest on reserves. Does that really change anything? Arguably no, other than possibly interest rates, but interest rates are near zero anyway. The trillion dollar platinum coin idea is an absurd exercise that people only suggested because US politics is absurd to begin with: congress orders the US government to spend money (via the budget bill), but then forbids the US government from actually implementing that spending in practice (via the debt ceiling and refusing to raise taxes). Congress wants to eat the cake and have it too. That is the reckless behavior here: putting the US government into a completely Kafkaesque situation. The way I see it, the platinum coin is mostly a rhetoric tool to highlight just how messed up the system is.