The intro to that article read like the author didn't do any research into the strip's history but just repeated press releases. Apparently Falk drew the original Mandrake character sketch, but neither he nor the syndicate pretended he drew the daily strip. Phil Davis is cut out of the story completely. Also, Mandrake started out doing full-on comic book magic but after a while dialed it back until he was only creating hypnotic illusions. A minor point but one that we'd expect to read in a Mandrake essay.
That aside, the story of the Mandrake TV show is fascinating. Someone at NBC must have been crazy to green light building an entire studio for the sake of producing an untested series! No matter how sure they were the show would be a hit, they could have got a lot more mileage out of their $750,000 by shooting on an existing stage in Hollywood or New York.
I agree that Mandrake as a character could have been more successful with better promotion. As the article implies, King Features was never too good about exploiting their properties in non-print media. One thing the article doesn't mention is that Mandrake, like The Phantom, seems to have been much more successful in other countries than in the US. I don't know if that's because overseas publishers promoted him more assertively or if something about the character has greater appeal to European sensibilities. As I say this I'm thinking of Tim Tyler's Luck, which never rose above a grade C newspaper strip over here, but was a giant hit in Italy and reprinted enlessly. Heck, Umberto Eco even wrote a novel about them!