I'm not sure what the tweet has to do with your question.
To your question: I'm referring to modern Western racism which is built on the scientific racism that became popular when there were economic incentives to explain why chattel slavery is okay when you do it with some people when it's otherwise not okay to do with others. This was downstream from a massive need for cheap labor in the colonies to produce exotic goods to export to Europe for profit.
I don't know why you're asking for an explanation of random unsourced and unqualified historical factoids (without mentioning e.g. which harems, which courts, where and in what time period, which seem kind of important specifics when using vague generalisations like "largely") - maybe ask whoever you learned that from unless you're "just asking questions".
But if you want a general answer to "why is there discrimination against groups outside the imperial core" mine would be that it is easier to justify an exploitative power hiearchy, especially one that subjugates the majority of "its own" people, if you declare outside forces as non-human or sub-human to prevent fraternization which might challenge your rule.
You can easily find this happening in sexism/"patriarchy": men are humans, women are different because they can get pregnant so they are more emotional, more deceitful, stupider, incapable of abstract thought, too easy to manipulate to deserve voting rights, more likely to cheat on their partners because they want the best genes for their offspring, naturally nurturing and caring, inherently better at social skills, inherently risk averse and unfit for leadership, etc etc whatever whatever. Or, as I already said, racism: white people are humans, Asians are different because they're clever but have no soul and operate like a hive mind, Black people are different because they're stronger but impulsive and child-like and must be disciplined to protect them from themselves, Arabs are different because they're deceitful and uncultured and only know how to steal and destroy and breed, etc etc whatever whatever. Heck, you can even find it in the trappings of "enlightened" critiques of democracy (or defense of capitalism, i.e. the centralisation of control of "capital"): us studied high-IQ people of wealth of course should get a say in things but most people allowed to vote are very stupid, easy to manipulate, only seek to reaffirm their biases, bordering on mentally incapable of managing their own life but also of course completely at fault for everything they suffer, etc etc whatever whatever. All of these are bullshit just-so generalisation that just happen to neatly explain why we (men, white folks, academics, people of wealth, etc) deserve to be in charge and anyone who isn't in that group not only does not deserve to have any say but it is in fact in their best interest for us to be in charge of their life too and if this just happens to benefit us immensely, that is only by pure circumstance and what harm does it do anyway if that is the case.
Also, I'm not talking about individual bigotry or stereotypes. "Scientific" racism existed to help perpetuate a system of power relations by justifying the ownership and subjugation of groups of people. Caste systems does and medieval European feudalism did much the same. "Tribalism" however is a red herring because in tribal systems, society is confined to the tribe itself and interactions between tribes are, essentially, diplomacy. Once society expands past a tribe, we usually use the term "nepotism" (or "networking" if you want a positive spin).