What's your definition of "diversity" here?
4000 years ago China and Europe did not look so different from each other! They both contained a mix of early centralized kingdoms, pastoralists, nomads, etc. and they both had a great diversity of people living in the region. However their evolution is very different.
Also you are already picking China as a single state, instead of the entire East or Southeast of Asia.
One could also argue that the European Union kinda, sorta constitutes a single "country". One that would become an instant superpower if we who live there wanted it to be...
But, on the whole, Europe tends to be more fragmented than China.
I'd say the geography has a lot to do with it. China has a couple of big rivers that are well-suited for long-distance travel. Europe doesn't. Europe is also a peninsula of peninsulas, often with mountains (Pyrenees, Alps) going across where one peninsula is attached to the next.
Less so in more northern climes where the weather is harsher and where we don't have much in the way of records, though we do have evidence of highly-developed cultures such as evidenced by structures such as Newgrange in Ireland, and evidence of trade corridors from Britain and Ireland down though the straits of Gibraltar to those Mediterranean kingdoms.
Unfortunately, I'm only about halfway through it, so I haven't gotten to the part where he tries to explain why this happened. But so far, it's quite good.
The "removal" of confuzianism happend within the last 100-150 years. Before that it was the backbone of chinese thinking for ~2500 years.
Europe being composed of 44 sovereign countries is a relatively recent phenomenon - like mostly post WWI. For most of history these countries were under the dominion of larger empires.
China did have much earlier centralization of government than other parts of the world, resulting in highly unified systems of administration, and most prominently, the shared logographic writing system. The administrative centralization of the past and present masks a lot of underlying diversity which persists to this day.
However this centralization was also contrasted by periods of fragmentation, like the Sixteen Kingdoms period: