I guess he really means "less complex harmonically". You're definitely right that a lot of modern music has features that would be unimaginable or impossible to achieve 40 years ago when this song was written, both through new tech and stylistic innovations. But I think his view is still very defensible if you focus on pop songs, singable vocal-led pieces that you might attempt in karaoke.

There was a long period where jazz influences were very big in popular music -- jazz itself was actually popular! -- so there were a lot of very harmonically interesting pop songs. I agree with him that that seems generally less true nowadays (thinking of big mainstream singer-songwriters like Adele and Ed Sheeran). But I'd be interested to hear of good counterexamples.