Tesla discovered something that indicates otherwise from a series of cars that caught fire - not even charging, just sitting parked (in some of the fires they had been sleeping in a garage for days). Their response was limiting SOC artificially below what was paid for, reducing charge speeds, reducing regen, and keeping the pack cooling system running for days on end. It's entirely beleivable that one of the "sick" batteries Tesla nerfed with batterygate would have caught fire if charged up and removed from a car, considering Tesla's responses to those fires.
The current hypothesis for this problem is lithium dendrites, they can be caused by fast Supercharging which explains chargegate speed reductions. Once they are formed, they can cause a fire at any time by slowly heating up the cell until it reaches an unstoppable cascade, but cells with dendrites are more likely to warm up at high states of charge, and that explains batterygate reducing capacity. Every older car was chargegate limited to stop more dendrites from forming and cars that had dendrites already detected had batterygate capacity limits imposed - all to stop the battery from catching fire randomly.
