The ship cannot be impounded. Also, the minister of defense also seemed to imply the ship had no goods on board.

Our country constantly monitors incoming ships to verify their identity. If a ship has no flag or a fraudulent flag, it is considered de facto ‘stateless’ and any state may enter and inspect it.

But chaining it up is more difficult. In the past, there were legal and practical obstacles to this: according to international maritime law, sailing without a flag is a criminal offense, but according to Belgian law, it is not. So boarding a ship for inspection is allowed, but seizure is not normally allowed, although the situation is legally complex.

Translated from the last paragraphs of this article in Dutch