The plots in the example you point to look to me like they can be generated without doing any contouring at all. I might be missing something, but they have the appearance being generated by having each point contribute to the coloring of the entire vertical and horizontal segment in which it lies. The different plots correspond to different rules about how that contribution is calculated.
Anyhow, back to gnuplot's current capabilities...
Some, but certainly not all, filled-contour plots can be created in two steps.
1) Use "set table" to draw individual contour sets to file
2) plot the resulting files, in order, using "with filledcurves"
The filledcurves code has been improved so that there are fewer problems with unclosed areas than there used to be. But it's still a problem.