Specifically a unique piece of Ming porcelain that was in the British Museum (how did it get there in the first place? The looting of the Summer Palace perhaps?
That's where most such treasures came from. Grand theft. So much was lost though, burnt or destroyed... and then that that survived sometimes got trashed in the Cultural Revolution. *SIGH*
Might the Chinese government have been trying to get it repatriated? Who cares? Not anyone in the story, that's for sure) was obtained by some corrupt means so that the British (again, who cares about China, right?) might not be upset by its disappearance and make a fuss, and then dangled as a prize to persuade a wealthy physicist to work on gravity control.
In 1941... there
was no China as a nation. There was the Kuomintang regime, but even there it was a gaggle of warlords who sometimes fought Mao's group, sometimes fought the Japanese, sometimes fought warlords who weren't part of the regime in the west (Sheng Shicai of Xinjiang comes to mind), and sometimes fought each other.
In 1941, thinking of China as a nation once more would be like thinking of the resurgence of the Holy Roman Empire today. Beyond imagination.
In early 1941, the Burma Road shut down, the railroad from Haiphong likewise, Russia a de-facto ally of the Axis, no longer supporting the Nationalists.. it was likely that what we call "China" today would be effectively extinct, replaced by a patchwork of successor-states in the Greater East Asia Co-prosperity Sphere a la Manchukuo. Any request for repatriation would likely be to Tokyo. If anything, Heinlein was a bit daring stating it was in the British Museum, and not in Berlin.
Tell me... if a Roman artifact was found in England - who should it be repatriated to? France - if it was made in Transalpine Gaul? Italy, since Rome is in Italy? The Vatican Museum perhaps?
I think it fair to assume that in this future history, there was no "Chinese government" as such. Any more than there's a "Roman government" today.
Then add the layers of culturally ingrained racism, cultural imperialism, treating the balkanised "central land" and its many nationalities as some kind of joke... yes, it's pretty bad. Insulting. Wrong. (Also sewing the wind, with the whirlwind to come in the 21st and especially 22nd centuries. If I was Chinese, I'd be seriously pissed, if you'll pardon the expression).
So although I feel you're wrong in detail, you're right in practice. The issue may have occurred to Heinlein - but never his readership. It, like "Huckleberry Finn" is a story of its times, and we should feel shamed that things were once like that.