It’s not clear from archaeology whether the earth mounds of Mississippian cultures such as Cahokia were cities or ritual centers. However, this used to be said of the Maya, too, and it’s now known that their temple complexes were part of very significant urban centers.
The difference is that the Maya build in stone rather than with soil, and that we have a deciphered historical record. In my opinion, the fertility of the Mississippi valley makes it likely that “mound builder” sites were cities, by an reasonable definition, but we’re unlikely to find sufficient remains to establish it.
Cahokia is well established as a major urban center.
The archeological digs on site are enormous. It is estimated that it held a population of 40k or more. This might be why someone would classify it as civilization since the tribal settlement cultures are generally much smaller.
I remember the days when some Mayan pyramids were thought by a few to be ceremonial centers. The Mayan culture itself has been thought to be a civilization for at least the last 60 years and AFAIK long before that.
The Mayans left writings on stone and that helps establish a timeline for them. To my knowledge no Cahokian writing has been found. Having built with dirt sometimes mound builders work is hard to discover.
Living up here in Western Pennsylvania, I’m familiar with the mound builders of this area and their sites are primitive compared to Cahokia. I don’t think anyone has discovered a major urban site near these mounds (they’ve certainly tried finding one).