Aha, well - while those things are improbable (really, nearly everything that happens in any action movie is improbable), they're not impossible or literally magical, which is really what Double Mumbo-Jumbo is describing.
i.e. the movie wants us to accept the existence of a could-never-have-existed rocket pack, so okay fine. If then at the beginning of Act III, a wizard showed up, or the archangel Gabriel or what have you, that'd be double-mumbo-jumbo - two unrelated inexplicable/impossible story points in one story. But there's nothing impossible or magical about Nazi's secretly hiding out in Los Angeles, or hiring local goons to do their bidding.
But it's certainly okay if you didn't buy the idea, that's entirely fair. Me, that's my favorite part of the movie, it comes right from the classic movie serials that Rocketeer was emulating. Just as Star Wars was partly an homage to the Buck Rogers serials, Rocketeer was an homage to RocketMan and Spy Smasher, etc.
And in those, boy, there were Nazi's and mob bosses hiding behind every bush in America. Which kept Spy Smasher pretty busy, as you can imagine.