Fluid may well have been burnt if the transmission was slipping. If they're going to claim the bands are burnt and it's slipping I would ask them to go through the FSM and report back the results of the stall speed test and the other tests.
Not sure the dates/mileage history of the prior flush, the accident, and this service so it's hard to say where your issue started, but if there's a period between the accident and this service where things seemed fine then it's hard to believe the tow truck operator destroyed the transmission as you would have noticed that right away once you were driving it again
BTW low fluid level will cause slipping, so I'd make sure the level was right before trusting that dealer If the last flush was recent it's entirely possible they neglected to fill the transmission correctly and they're trying to CYA.
-2021 Heritage. 27000 miles. Standard maintenance by the toyota manual, I did my own oil changes.
-Accident happened June 30th. From accident to drop off, the vehicle was only moved in neutral to be winched onto the tow truck, and then winched off the tow truck.
-My collision rep told me that toyota drove the vehicle from where I had dropped it to the service bay after "installing a bypass around the oil cooler".
-I picked up the repaired vehicle from service on October 6th. They had "flushed the transmission" during the repair of my vehicle because the engine was throwing codes related to the solenoids and because they had to replace my oil cooler from my front end accident. No transmission slip noted by anyone during the insurance repair nor during my test drive before accepting the vehicle back.
-I drove the vehicle the weekend of 7/8/9 October and noticed a "ticking sound" at low speeds like something was in my tire. checked my tires and nothing there. I never felt anything different from previous driving during this 3 day period where I put about 150 miles on the vehicle via highway driving.
-Returned the vehicle to toyota Carlsbad collision center on 10 October to diagnose the ticking noise.
-Called on 11 October to say my transmission was slipping. This "slipping" was confirmed by two master techs at the toyota service center, both have 20+ years of experience. Collision GM also said it was very noticeable.
-They're dropping the transmission pan today, expect an update this morning/afternoon.
I'm trying to say that this should be a warranty claim not an insurance claim.