Popular music ebbs and flows in it's harmonic complexity over decades. Certainly in the last century of popular music. One of James Brown's innovations in the 60s was reducing jazz and RnB music down to one chord for an entire song. On the other hand, 70's songwriting was harmonically rich, with lots of adventurous modulations, modal interchange, and extended chords (7ths, 9ths, etc).
We're now coming out of a two decade run where hip hop has been the dominant popular music. A form which, generally speaking, tends to be harmonically "flat" (ala James Brown) and simple in form. For songwriters who are looking to differentiate themselves from their peers a move back into harmonic complexity is a worthy move and low hanging fruit in a musical era where few have been making use of it.
Compelling music has always been, in some sense, about setting up expectations and then subverting them. The art is in the balancing of steady patterns and novel events.
The example in the article seems to hint at "harmonic surprise" referring to a simple genre shift:
>With its stark, sudden shifts between choral melodies in major chords and menacing percussive elements drawn from the trap subgenre, the song constantly defies the listener's expectations throughout. That's why "This is America" also tops the list of pop songs rich in so-called "harmonic surprise," or points when the music deviates from listener expectations.
I've also noticed that most pop music today (not to blame any genre) is rhythmically flat. Not many examples of 'Take 5'. Same drum samples used throughout the whole of 5-minute pieces. Lots of studio processing to make the result seem more like music. Which it isn't.
It isn't the machines' fault; it's the 'producers' manning them. Drop a hit now'n then. Throw in an extra one. Too busy cranking out the candy? I just call it: lazy, lazy, lazy. And very, very boring.