Exactly ! (...and thanks for its meaning).
The funniest part is the title also says : "You Read That Right" ...well, no ! ...I didn't since it was unreadable for me :-O
I get the term after reading the article, I've just never heard it described that way before. Their own figures say union membership rates in the article which seems much clearer to me and has the same number of words as "union density rate." Density seems to imply something more geographic or spatial, just seems like an odd way to describe it.