Same way as everywhere else, ruthless exploitation of the poor and the friendship of a superpower that saw them as a useful proxy.
Do you genuinely think that the only way anyone gets any wealth is by exploitation?
No. But if you think "any wealth" is a synonym for "capitalism", you're very uninformed on economic history; there was plenty of wealth, even unequal wealth, before capitalism ever developed or was defined.
But if the question is whether
extreme wealth disparities are indicative of exploitation, that strikes me as tranparently obvious. You shouldn't need Marx to explain something so basic as the fact that if Reggie and Reginald both work thirty hours a week, but Reggie "works for" Reginald, and Reginald makes 2000 times what Reggie does, that Reginald is probably on the take somehow - whether by scheming or heredity, they are in a relationship where Reggie does not get compensated for his work in the same way as Reginald.
If you think that's super cool and entirely justifiable, then congratulations -- you're a capitalist! Maybe you think Reginald must be two thousand times smarter and more valuable than Reggie, that The Market would step in to magically correct it if it weren't, and it's sheer coincidence that one thousand years previous Reggie's ancestors were uncompensated serfs while Reginald's were wealthy churchmen who transitioned to banking when the gods of our society changed. Great, if you like. Nothing to stop you from drinking the Kool-aid(tm) if Kool-aid(tm) is your beverage of choice. But thinking that it is good for the wealthy to extract labor from the poor doesn't make it any less
exploitative That's just grammar and logic at play.