The difference is between observable facts and a wackadoodle conspiracies.
That America is shifting towards a majority-minority population is not a controversial observation. It is very broadly accepted that, due to immigration from predominantly non-white and/or Hispanic regions of the world, the relatively higher fertility rate of non-white and/or Hispanic families, and the increased number of mixed-raced families compared to previous generations, the proportion of Americans who identify as "non-Hispanic white" is going to drop to below 50% in the next few generations, even as the absolute number of non-Hispanic white Americans remains stable or increases, and the number of people with at least one non-Hispanic white American parent (i.e. people of mixed race, in which one of those components is white) is certain to increase. This is a long-term trend that has been fairly self-evident for decades. Hamid's observation was that undermined the electoral position of the Republican Party, an observation which has been made many times before, and that he found that to be personally satisfying. He was kind of a weirdo about it, but his avatar was an oil portrait of the Shah of Iran, it's to be expected.
The "great replacement" to which Cloud referred is a far-right conspiracy theory, which maintains that white people are not gradually shifting from a majority to a plurality in certain parts of the United States, but that they are actually threatened with extinction on a global level, and that this is an imminent threat likely to resolve itself within the next few decades. The mechanics of this "replacement" are not impersonal demographic trends, but a general trajectory of cultural and social degeneration, with specific blame directed towards homosexuality, feminism and multiculturalism. Very usually this is a deliberate or semi-deliberate campaign by globalists and/or Marxists and/or the Jews to produce more pliable populations. It would be strange, and in my experience unprecedented, for somebody to buy into the theory to the point where they believe that white people are threatened with eminent imminent, but then decide this is nobody's fault in particular. Posters who subscribe to this later theory therefore tend to go off the rails fairly quickly, as their demographic jeremiads degenerate into openly homophobic, misogynistic, racist and anti-Semitic bile, and that is the sort of thing CFC's moderating team crack down on pretty quickly.
