It is not correct to compare Polish-Lithuanian Commonwealth with USA demographically.
PLC had very tolerant views towards minorities that moved to Poland or were living in their native homelands Eastward (until Szlachta's power went to their heads) and allowed minorities to carry on living as they would be in their homes, with total freedom.
However, Catholic Church was still first and foremost authority in Poland, which did draw divides between peoples of different faiths. They lived separate from each other, segregated, forming what is called today "cultural salad".
In USA, integration is one of central agendas of minorities and there is etonym "American", which is citizenship that anyone can claim on US soil, while in PLC everyone was either Polish, Jewish, German, Ruthenian or Lithuanian, and not one nationality as Americans can claim. This means there are no barriers, forming what is called today "cultural melting pot".
Difference between two concepts? Short summary, "cultural salad" (PLC) means many different cultures living in country seperate from each other, while "cultural melting pot" means many cultures integrating together under common citizenship/"nationality".
Also, it is unfair to call Poland's homogeneity "unnatural". Back then PLC was empire, which by definition is multi-ethnic, while today it is nation state of Polish people. If anything, it should be honogenous, like other nation states worldwide.