I think reading between the lines a little bit should indicate that because 50%+ of professors are adjuncts means that universities are proactively attempting to keep professors with that status. It means they can pay them less both in salary and in benefits. Hence why Duequesne would want to avoid unionization of it's adjuncts.
I don't know the statistics of adjuncts who have full time jobs as well, so take that with a grain of salt.
That statistic of 50% professors being adjunct is not cited, and from searching online appears to come from the SEIU, which is a group who's goal is to unionize adjunct faculty. The report it self is not available, and it has been quoted in articles as "part-time faculty held 50 percent of teaching jobs at colleges" [1] and "adjuncts held 50 percent of teaching jobs at colleges" [2]
Neither of those statements are the same as "Adjuncts now make up well over 50 percent of the faculty at colleges and universities." as stated in the article.
I don't know the statistics of adjuncts who have full time jobs as well, so take that with a grain of salt.