"Now any repair job can be 'duck soup' for you! Take it from me, Sean Penn. I'm from the future!" I don't like what MoToR is implying about Future Sean Penn. It seems thay mean to say that repairing old cars is so easy, Sean Penn could do it. Did they even see Mystic River? When Penn is (was) born, MoToR will have regretted their pejorative attitude about his intelligence.
What's he doing with his hand? Maybe in years past holding your hand in this way was a gesture of reassurance? Maybe he's having his elbow reattached just out of shot? Maybe he's shielding his eyes from flying metal debris as he mills the cylinder walls of a Hudson with his other hand... while simultaneously doing an endorsement? This book really must make car repair "duck soup"! Future Sean Penn should be wearing goggles while doing that, even if a book is making it "duck soup" for him. He'll need his sight to read all the scripts he'll receive in his career as a successful actor, once he's exists.
Speaking of anachronistic phrases, there's another one in the ad copy: "dope".
One million years ago (in the 1940's or something), "the straight dope" meant information, especially from a reliable source, or of a secret nature. If someone was telling you how to do something, they were giving you "the straight dope". The word origin is originally Dutch "doop" for any thick gravy or dipping sauce. It came to be used for liquid opium, and after that, for drugs in general. The use of the word to mean "inside information" is probably from horse racing, and being tipped off before the race which horse had been drugged to improve performance.
Lastly, The Straight Dope is a question and answer column that's been published in the Chicago Reader since 1973. Readers send questions to writer Cecil Adams and he researches the answers to interesting questions like "can fire ant bites kill you?". Always fascinating. There are much worse ways to spend a lunch hour.
Good luck finding out why "duck soup" means "something easy". Apart from the Marx Brothers film, there's only speculation.