Generally speaking, a coating will conform to the substrate as a function of both viscosity and layer depth. You are right that a more viscous coating is going to be easier to apply and smooth, but it still won’t be particularly smooth unless you did the pre-work to get it there.

That person sanded their print first. There are also some filler-primer paints that will fill the layer line crevices for you, but you still need to sand the surface before applying anything.