In the Wikipedia article on Chinese City Walls, they mention the size and thickness of Chinese Walls as a potential reason for the lack of development for large gunpowder artillery pieces in China. It mentions that Chinese city walls often reached 10-20m at the base and 5-10m high, while 2.5m thick is considered very thick for contemporary European walls. It is also much more sloped than contemporary European walls, allowing it to better deflect projectiles.

To support its claims, the article points out that in the 1490s, Florentine diplomats considered French boasts of being able to breach 8ft (2.44m) thick walls with their artillery to be ridiculous and delusional. Apparently mid-20th century batteries even had trouble creating breaches in these walls, the article giving two examples during the Second Sino-Japanese War and the Chinese Civil War at Nanjing and Tatung respectively.

However, these claims seem to run counter to what happened during the Battle of Xiangyang in 1273, where the Mongols famously captured the city with the help of counterweight trebuchets imported from the Middle East. The walls of Xiangyang are, according to this article, 6-7m thick, which is significantly thicker than what's mentioned above in the example with the Florentine diplomats. Am I to believe that counterweight trebuchets are actually more effective at breaching fortifications than gunpowder artillery created 200 years later? What's to account for this discrepancy? Or is the Chinese Wall Theory just bullocks?