It’s the load’s ability to keep going in spite of the car. Truckers call it wagging the dog. I have no doubt that you have had success with towing that load. Everything is okay until it isn’t.
The 450 was never approved for towing by smart. Despite that, it was TUV certified for just under 800 pounds. The 450 was a less powerful, more fragile, and smaller car. Certainly if a 450 can safely handle 800 a 451 can handle a little more. We also have a 453 owner on this forum who routinely tows 1,800+ pounds.I've even had to emergency stop a few times and the car handles it like a champ.
In this time I've also upgraded my brakes and am rolling on tyres designed for significantly more weight than stock.