I wonder how much post-processing they had to do?

There's a few different actions. First, since it isn't developed the paper effectively acts extremely slowly. Whereas normal photographic paper would be around 6 or up to 20 ISO, doing it this way is almost immeasurably slow. Getting similar results without a pinhole and just something layed on top (photogenic drawing), it would take several hours in direct sun to get a decent image. The next contributing factor is that some papers "print out" easier than others. The hard ones will take hours in direct sunlight, while the easier ones might take "only" 30 minutes or so. The final factor is that pinhole is a very slow and low contrast method of exposure. Silver halide paper suffers from "repricocity failure" where basically during a very long exposure, the material becomes massively less sensitive. This is why film is so hard for astrophotography. A 100 ISO film over several hours can become something more like 1 ISO. Either way, the end result is that the paper gets very slow and gains a very wide exposure latitude, nearly impossible to over expose.