Oh, so the creep has a prebaked set of spread states and not generated at runtime in that shape. I see.
Viewing post in Let it Grow jam comments
Yes, it's prebaked here, but realtime generation is somewhat similar. You can use a shader with a stencil buffer to form the shape of the creep, which will be filled across the ground plane behind it. In this case, the build map will be filled with the shape of this stencil layer instead of textures. It takes some time to create and set up, so we don't use this technique within jam time limits.