It's absolutely about China
They obviously just thought Japan had better imagery when they decided it would be the title track, as it was originally called The Great Wall.
Japan is an island country, how can invaders ride in
From Bruce's interviews it's said to be a mix of Chinese Great Wall + Game of Thrones (Hadrian's) Wall. From the lyrics it looks like China, yet not 100%:
Set the beacons afire for them all --> Probably universal reference, though there were plenty of beacon towers' systems at various locations in proximity of Great Wall. Also Lord of the Rings comes to mind.
Call to arms all the men far and wide
Have to fight now for
dynasty pride at stake --> Looks like China.
The invaders repel from the north --> Looks like both China & Game of Thrones.
Keep out
nomads who came from the
plains --> Looks like China big time.
Northern grasslands awash with them all --> Same as above.
Blocking the
tribes that invade from the south of us --> Now that doesn't look like China, Dothraki comes to mind and even that doesn't link very well.
While I agree that Japan appeals better to Western audiences and that could be one reason, from various interviews also seems that Harris wanted an Eddie with a sword. Now since historically Japanese swords had the best of reputation, as the peak of craftsmanship, they naturally gave Eddie a such sword, thus the Samurai -Eddie, so Japan.