Not to belabour any specific point you're making, but Rome had three classes.
* High-Romans - elected officials, ranked soldiers.
* Romans (Or, Honorary Citizens in rare, exceptional cases).
And...
* Slaves, which were the result of conquest of those diverse lands.
Ancient Rome, especially during the Republic and early Empire, wasn’t ethnically diverse when it came to fairness and equality at all.
Citizenship was originally reserved for freeborn males of Latin descent, and while it expanded over time, it was usually about practicality and control, not inclusivity.
Conquered peoples, like the Greeks and Gauls, were brought into the empire as subjects, not equals. Slavery was a huge part of Roman society, with enslaved people often coming from different ethnic groups, but they had no rights. Even when enslaved people were freed, they still faced heavy discrimination, and the upper classes worked hard to keep Latin culture and traditions dominant over everything else.
It wasn’t until much later, around 212 CE, that Rome started to look more diverse in a way that also brought some measure of equity. That’s when Emperor Caracalla granted citizenship to all free people in the empire, regardless of their ethnicity. However, this wasn’t really about fairness—it was more about making tax collection easier and keeping the empire running. Even after that, inequality stuck around, with most power staying in the hands of the wealthy, Latin-speaking elites. While non-Italian emperors and leaders became more common in the later Empire, this shift was more about necessity as Rome struggled to maintain control, not a true embrace of diversity or equality - and if anything, something that people who are not fond of diversity can point to as a cause of Rome's fall (not that it actually is.)
It's not just about the various social classes, but rights of outside groups as well, and the ability to move between those classes. The linked essay has the details.
Charitably, I think they mean we are starting to see explicit alignment between government and the ultra wealthy, which definitely is a threat to democracy. You can either do what is good for the majority or what is good for the 0.0001%.
