If you have a double conversion UPS that is complaining about less than 100W deviation, I would recommend you check the UPS for a component that is out of spec or on the way to failure.

The concern isn't the average rated TDP. It's the high Di/dt (change in current over time) transients of certain workload profiles cascading through the various layers of switch mode power supplies. Every layer of power delivery has some reactivity to it. I'd agree this would be no problem if all our power supplies were purely linear (and massively inefficient).