Good question, in one of history’s more bizarre episodes, the Chinese turned inward when a new Dynasty emerged, they had their fleet burned, the same fleet that did travel the whole of the Indian Ocean, they had massive ships at the time.
Funny old world, China decided that they were the whole world and cut off contact with the world in their perceived perfection...
This was in large part due to the influence that eunuchs had over the emperor at the time. Many scientists and scholars were banished, if not outright killed, for challenging the orthodoxy of the imperial court. Many of their texts were supposedly burned.
It is quite possible that some ideas migrated from East to West through cities like Venice. In 1987, I attended an exhibit entitled China in Venice; some of the toys that mariners brought back did look similar to some of Da Vinci’s later stuff. Like all great inventors, many of his ideas probably came from a number of sources. To claim they were all Chinese is overly simplistic.
The two words in Chinese, jung guo, actually mean Middle Kingdom, which essentially backs up your second statement.