I believe there was little support for SPDY on CDN when thethemefoundry wrote that article. You would really want to do both.
Browsers only use 1 SPDY connection per host. The image proxy would basically eliminate mix content messages. This video is what I have found the best at explaining it. SPDY is basically HTTP 2.0.
SSL is basically needed to keep networks from blocking its traffic.
http://www.youtube.com/watch?v=WkLBrHW4NhQ