Not familar with the owners but they likely just forward traffic from other domains to that one to build the traffic.. Not saying it's wrong or right.. Just saying if they have 10 domains doing 50,000 hits a month each.. They just forward that traffic and make it look like they have a monster..
The keywords they used on this one might be worth more $$ then the ones they would have on the domains they're using to forward.. Anyone who has 500,000 hits a month or the ability to produce that kind of traffic likely has a strong connection with their acct manager at Sedo and I'm sure the matter will be given the proper attention.
I'm almost positive this is how they did it.. Those with names on Sedos top traffic list often own HUGE portfolios of PPC traffic earners that they simply forward to one or more names they need to beef up the traffic on.