To me, this experiment only shows that people can keep track of time in their dreams when they actively try. I don't see it proving that dream time and waking time are always synchronized.

Lucid dreams can seem like they last a long time. I have had dreams that seemed to last hours, days, years, lifetimes. It is obviously an illusion, but our perception of time is an illusion. We don't have a constant or reliable perception of time in waking life, so it only makes sense that dreams would no better. I can count off seconds sitting at my desk at work, and the day would seem to go by horribly slowly. If I got myself involved in an interesting project, hours would fly by without my noticing.