I was also interested in that comment. I doubt it is "true" as a general statement - after all, there are many factors to generating wake turbulence.
What is true is that the 757 is towards the top end of the weight category for a "medium", and also has caused a number of wake events. As such, some operators choose to treat it as a "heavy" and require 2 minutes after it's departure etc.
I stand by to be corrected on the overall statement that a 757 generates more wake than a 747...
NoD