Calling it a "postprocessor" is weird. It gets parsed into CSS before the browser sees it, so it's a preprocessor. If something parses the CSS out from the browser, it can be a postprocessor, ala -prefix-free.
I really love the idea of authoring as if vendor prefixes never existed and letting software fix it up for you. This is what Autoprefixer does. It might just use that internally?
The whole point of calc() is that you can do things that are impossible to calculate before the browser is involved (e.g. 100% - 20px). It should be more clear that this can't preprocess calc() into working in browsers that don't support it.