The fact this was released by Hanna-Barbera is a bit odd (too bad they didn't do a crossover with The Flintstones or Scooby Doo). Although HBR doesn't really do this, the Canadian issue on Pye makes it appear like it's a soundtrack to the TV show. Other than the theme song, none of the other tracks connect to the show.
Should be noted that this is not a soundtrack album for the TV Show. The fact it includes Johnson's theme gave the label an excuse to put the Avengers branding on it, but otherwise it's just a collection of instrumental covers.
Considering how well known Nellie Bly was I'm surprised the writer of this got away with using it as a pen name (that said, the original Nellie was a pseudonym, too).