For the folks who stumble on this, a correction to the information in the article:
Procmail has supported Maildir natively since version 3.14. Since the latest version is 3.22, stating that it only supports mbox is a bit disingenuous or indicative of shoddy research. In fact, the version of the "mini-FAQ" linked from this article is NOT the same one linked from the procmail.org site!