I'm assuming you are still losing power. Some power is taken out via the USB and put back into the computer, and some is use to power the device like normal. Since the power put back into the computer is not larger than the power being taken out, there is still a net loss in power over time.
Normally I wouldn't comment on a shitpost, however...
Not only is there power drain, but the device will think it's plugged in and actively disable power-saving features (someone correct me if I'm wrong).
The problem is not to do with the 'power put back' being 'larger', it's to do with efficiency. If charging and discharging a battery were 100% efficient and there were no losses to heat through the rest of the system, then there'd be no discharging, but, yes, due to the laws of the universe, this will always cause a net loss.
But if it is 100% efficient, the computer would still be using power to run.
if nothing else there is the led on the charger. (but of course there are other things like heat generated by charging, I would assume)
I still bet this is the reason that Apple didn't put a second USB-C port on the new MacBook. It would create a perpetuum mobile and Homer Simpson would not approve since "In this house we follow the rules of thermodynamics!"