I'm not sure I've heard that argued, and would generally agree that a President can own interest in foreign companies. However in an effort to avoid clear conflicts of interest or an actual emolument from occurring, it would be wise of them to divest of them. Otherwise a foreign state can use it against them in some way, either to attempt to extort, or to visibly promote it to create the air of conflict. And then the President would be unable to do anything without the taint.