1. I am curious, how many of the 680k songs are unique? It is rather uncommon for massively successful songs to only have one version of tabs out in the wild, so I am curious how many songs individual songs were counted multiple times.
2. This analysis only looks at guitar tabs or instrumentations there were transcribed for guitar. Chords can be made with more than just one instrument, thus that missing 7th note could actually be played by another instrument not included in the tabs.
3. As music progressed from the pre-jazz era to modern times, it became more common for people to play an instrument, like piano or guitar, while singing at the same time. Obviously there are exceptions to everything, but often times guitar pieces are simplified if the guitarist is also singing for practical reasons.
4. Music has also become more accessible as time progressed. It would be hard for an average person to learn the organ or hurdy-gurdy without access to one. It's much easier to acquire and learn piano when it can be a 4 inch thick plastic keyboard on a stand.
5. People tend to have a warped concept of the history of music. Pachelbel's Canon in D is by no means a complex song and has stood the test of time. Music through out time has also served different purposes. Hell, go back to Ancient Greece, Gregorian chants, and Medieval music. Those various time periods were not generally fully of complexity either. I would argue such times were generally less complex than modern music.