I found a letter to the editor which stated those facts but can't find any back up for them. Why would someone work for $20K per year? They could get temp work for $15 to $20 an hour or work in the private sector in their field.
I'm not sure what Kron considers a full teaching load, but for our fair university, it's 12 quarter hours, three quarters per year. And it may vary by department or by faculty credentials, but I make about $650 per credit hour adjuncting, so for a "full-time" teaching load, it's about $23,400 a year. If you figure an hour of prep time for each hour of class (and, it has been my experience, that after teaching a class a couple of times, that's about how much time I spend prepping), it works out to about $32.50/hour.