How much AC Power will a DC powered (50 W DC) Device need if we operate it will an AC to DC Converter?
If a device needs 50 W, it needs 50 W. If the input waveform is shaped like an ice cream cone, it still needs 50 W. Think of the simplest device: A heating element. A simple resistance that needs 50 W to operate.

No matter what you do, you can't dream up a device that operates this using less power. If so, you could extract the power, feed a fraction back, use the rest, and now you have a perpetual motion machine.

It is likely that you or the person who told you this confused current with power. It is generally true that a device normally operated on DC will draw much less current from an AC outlet using a suitable adapter.