You 3d print molds with a separate 3d printer.

It feels implied from the video and the pictures and the opening paragraph

> If you own a 3D printer, you've got hard plastics covered. . But what if you want to make something soft or squishy, like grip pads or a gadget enclosure?

Print in TPU? There is a limit to how soft one can get, and the finish is not so great (stringing). But fantastic for very tough semi-soft functional objects. Ninjaflex Cheetah and Armadillo are my favorites, but generic TPU also works. Direct drive extruder recommended, though I manage on my cheapo Bowden fed Ender 3.