The bike was definitely badges as a De Rosa when Ken bought it. In fact, 43bikes has photos of it from the original ebay auction:

http://www.43bikes.com/derosa.html

I remember when the bike sold - for $3450. I would have bought it if it had been bigger (of course, someone may well have beat me to it had it been bigger). My supposition is that if Ken had emailed De Rosa and been told the bike wasn't authentic, he would have demanded a refund from the original seller, rather than spending money to repaint the bike (again) as a De Rosa and sourcing some (very) rare parts for the rebuild. The metal headbadge would have been tough to fake, too, I'm guessing. I'm betting De Rosa remembers the bike. He began building circa 1953, and even by the early 70's was building only a couple of hundred bikes a year, give or take. Ken could easily have resold it as a De Rosa without going to what was apparently considerable trouble.
Edit: It also seem unlikely to me that someone looking to fake a De Rosa would have chosen a bike that looked so much like a Cinelli, the features of which are so distinctive. The cost to create the fake would have added little to what the faker could have gotten for a Cinelli from the same period anyway.