I am not sure about September but we were at Typhoon Lagoon this past Saturday and it was crazy crowded with locals. We arrived at 9:30 AM (30 minutes prior to open) and the parking lot was 2/3 full. People were backed up at all of the bridges similar to a MK Rope Drop Crowd and it was a running stampede to any chairs with shade. People were literally throwing their belongings onto chairs and rushing to the slides which all had long lines almost immediately. By about 11:30 AM the lazy river was so packed that every tube was in use and we got stuck multiple times just because there was nowhere to go.
When we left around 1 PM the parking lot was overflowing past capacity with people parking in the grass.
Personally, I will never go to TL again on a Saturday unless it's pretty far off-season.