The only thing it missed is the simple reason why it happened in Sweden. As someone who lived in Sweden for a while, it's right in their culture and language. Swedish itself is very rhythmic and musical. It's the reason why Swedish-English speakers in the 1950's and 60's sounded like the were singing or yodeling when they spoke in English. Also melodies and singing are a huge part of the culture going back hundreds, if not thousands of years.
But excellent overview on how it happened. One of the things many don't know was that Benny and Bjorn from ABBA had already been record producing management for 5 years, under Stig Anderson, before Waterloo ever became a hit. So one of the main reasons why ABBA became so prolific was that Benny and Bjorn, with the help of Stig and sound engineer Michael B. Tretow, had the expertise to produce and turn any melody into a good song. They didn't need the help of anyone else, as they were one of the few human beings who could take a song all the way through the process. Enjoying the long drown out process of producing a tune into a hit, as it takes a lot to get through the winters in Sweden, is also a big part of Swedish culture, and the asset that became the backbone of why Sweden became such a great record producing country.