Re: 1983 synthesisers!
I think the study is showing an American bias.
RTFA. The study tracked changes in the music in the US Billboard Top 100 chart. It's not "showing an American bias" - it''s about popular music in the US.
Similarly, it's not about what was "well established" in particular genres. It's about what was in the Top 100. And while Pink Floyd, for one, were indeed both successful and influential in the US (Dark Side of the Moon will apparently be on one of the Billboard charts until the heat death of the universe), such successes were not sufficient to define the overall shape of popular music in the US.
It's a shame1 the study is unrelated to your concerns, but them's the breaks. You're welcome to do your own statistical analysis of other musical trends in other places.