This kind of mentality goes beyond countries or cultures, but also affects differences between generations, genders, sexuality, etc. It is far more convenient to explain away different behaviors as "It should be expected, she's X" (or the flipped "I can't help it, I'm X") than trying to work out and overcome differences on an individual level. We're all guilty of it to some extent as it helps build a sense of identity (notably among minority groups) but the normative aspect of it is very harmful.
Similarly, immigrant groups tend not to integrate, critics from foreign countries really are usually misunderstanding a large number of important points, and so on.
Between any two cultures, there is a huge amount of overlap. But nothing quite coincides, and the differences end up mattering a lot.
Your 12th century stories are interesting and all but do not reflect the reality of the world we currently live in. If you live in a large city and have a medium-to-high salary, which is a reasonable assumption to make given HN's demographics, it is in fact easy to get acquainted to other cultures by traveling often (possibly living abroad), learning other languages and/or befriending (or dating) foreigners. Much easier than used to be a couple centuries ago, in any case. If you do, you will find that deep down beneath the veneer of culture humans are very much the same. I don't have data to back this up, this is only from my mere experience.
Pretending that cultural differences don't exist is harmful and condescending; acting as though they were insurmountable and irreconcilable is downright dangerous and leads to disaster.
Damn straight! Beijing just wants to rile up Hong Kong into defeating themselves, presumably so someplace closer to Beijing becomes a more powerful trading post. No other party has a motive to turn HK against itself like this.
Sarcastically speaking, such stereotyping is typical of arrogant Westerners.