The USB-PD standard covers charging up to 100W (20V/5A) over a USB-C connection, but it's unlikely your laptop's ports can output anything other than 5V.
Dell's page appears to have some editing errors--my guess is that the sentence you quoted should say "can supply 100W of power for charging
or power a 15W device"--but here's a graphic from the same page that makes more sense:
Thunderbolt 3 supports 100W input (i.e. for charging your laptop on those laptops that charge over USB-C), but can only output 15W (5V/3A) for charging or powering devices.
Unfortunately, that means you'll have to continue to use a brick to power the monitor if it needs 12V.
It looks like there are monitors available with a low enough power draw that they can run off nothing but a Thunderbolt 3 or USB-C displayport, e.g. a couple ASUS ZenScreens