While the "debt" drawback implies that simply having the coin doesn't put someone in debt, the rest of the information of the Ebon Mint sort of does, even if it's a bit of a stretch to apply it to the coin itself. Specifically this part:
"The black coin is a pseudo-coin. Yes, a package of magic and power, but also likely a conduit back to whatever power is behind the Ebon Mint. You can spend it immediately or hand onto it for later. Unlike the other coins, the Black Coin does not reduce in effect once your transformation ends. It can be held onto indefinitely, a card up your sleeve for the perfect moment. Or maybe you're the self-sacrificing sort - hoping to keep the coin out of the hands of those who are less principled than yourself. Either way, it will remain in your possession until you spend it or give it away."
That highly implies that it can't be stolen, at least not by anyone who doesn't know the secrets of the coin, ie someone directly from the Ebon Mint. Combined with it being a coin at all, currency in the magical world, even if not like a normal one, it can be implied that it has intrinsic value even before being spent. The "Debt" drawback also directly states it's a loan, and the overall "Drawbacks" portion of the Ebon Mint implies that as long as you were chosen to get a coin at all, they're going to consider you in their debt via blackmail and small requests. I just sort of bundled all that together into "the coin puts you in debt that is normally impossible to actually pay off."