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).
Thanks for the correction: I haven't followed archaeology in this area. A population of 40,000 would absolutely be "civilization": you can't call that number anything but a city!
Have you visited a lot of Mound Builder sites? I haven't. We could have got to some from Tulsa easily, but we went to modern American Indian sites instead.