1) First off, Anthony Roberts is the biggest retard in the world of AAS and has 0 credibility. There is a sticky on this site explaining why.
2) That article is 5 years old, so a lot of things have changed since then (due to more recent studies and findings).
Other than that, yes, I believe that Winstrol does compete for the progesterone receptor, and that this might partially explain why it dries out the joints so bad. However, it doesn't actually lower progesterone, just compete with it at the receptor. And this is still largely speculation and theory based on this pub-med article:
http://www.ncbi.nlm.nih.gov/pubmed/8079819
You also have to take into account that Winny is 5a-reduced and binds tightly at the AR, so it antagonizes estrogen effectively, which will dry out your joints by itself to a degree.