The computer is a Studio XPS. When I bought it some 3-1/2 years ago it was Dell's top of the line desktop. I plugged in into my existing UPS and did a test and the computer shut off. After some research this is what Dell told me, and I had to buy this higher-cost UPS to work with it. Of course you may be right, perhaps it's just a quirk of a poorly designed or cheap power supply and Dell was blowing smoke. Fact is, however, the system has been working without a single glitch for all this time.
Dell is not blowing smoke! I have the same experience with a Dell Precision T7400 workstation, the UPS from APC didn't switch fast enough. I only found that out on a rare occasion that we experience an interruption of power-supply in the Netherlands, caused by local digging by another utilities company which damaged the sub-terrain power lines. I replaced it with a UPS from AEG, this time tested it by cutting the power myself, and that triggered the AEG unit without causing any problems.