In the fictional 2003 book The Next Continent (p. 241-252), set in 2030, the Moon Treaty is used in the UN International Court of Justice to put a commercial project at the south pole of the moon on hold. The project was being conducted by a Japanese company, and while Japan never signed the Moon Treaty, it is argued (in the fictional plot line) that customary international law applies to the situation. Supposedly, the Moon Treaty bans use of the moon's resources for commercial purposes. The majority of their tourist-oriented moon palace is made of lunar ice, so in-situ resource use is unavoidable. In this case, the plaintiff is the US government, whose competing NASA moon base is scientific and not for profit.
Legally, how realistic is this? The author seems to have done his research. I want to know if these arguments are legally sound, legally weak, or laughable.