Chinese units can’t leave China. However, they may enter Hong Kong.
Since in many respects, China is played closer to the historic reality, I am not sure that I would agree with that. In 1944, Chinese units were moved into Northern Burma as part of the Allied offensive to clear Northern Burma and get the Ledo Road into operation. You also had Merrill’s Marauders in operation there as well. Once the Northern Burma area had been cleared, the British-Indian forces proceeded to clear the rest of Burma, using a fair number of tanks units in the process once they got clear of the mountainous terrain. This is one area where it would be nice to have some terrain effects, as the Assam-Northern Burma area and the Assam-Burma border is some of the worst terrain in the world to fight in or try to conduct military operations through. It is not totally impassable, like the Sahara Desert or Himalayan Mountains, just very close to it.
Please recall that the game rules are probably to facilitate game play moreso than to ignore the historical reality of what happened. Maybe you can incorporate a house rule that says something like ‘after round 10, the chinese units are no longer restricted to China’
Substitute whatever round you want, I just chose 10 for an example.