I didn't see anything in the article that implied the rogue waves were proportional to the average. In fact the thing that makes them rogue is that they are many standard deviations away from the average.
This is not the first article I've read on the subject, it's my understanding that rogue waves are built from regular waves, therefore they're proportional - just many standard deviations away as you say. You can get a 20 meter wave in the midst of 10 meter waves, but you're not going to find that in 2 meter waves, rounded to an appropriate number of decimal places.
