When I think of complexity I think of unreconcilable elements that force the transposer to make tough decisions ("intentionally putting their own stylistic spin on it").
- Rhythmic patterns and variation
- Interplay between instruments
- Instrumentation and arrangement
- Structure
- Vocal style
- Lyrics
- Recording and mixing
By these metrics (and the ears of 99% of its listeners) it's a more or less generic 80s adult contemporary song. Yes it has a weird chord progression. Would it be more complex if it couldn't be boiled down to a series of chords?
That’s what the word “pop” is being used for in “the most complex pop song ever”. Rick Beato is giving an example of a literally popular song, one that somebody suggested they perform an impromptu concert because it was in the charts at the time, and which sounds totally mainstream, and yet has a very unusual chord progression.
And I think “music theory” in this context is basically jazz theory, where a song is boiled down to what you’d see in the Real Book - melody line, chords, and a description of the tempo and groove. Unless you’re doing big band, instrumentation is one of the standard small combos, or just whatever musicians you have to hand. Which again is reasonable in the context of “here’s a song we tried to busk in a scratch group, and it turned out to be crazily complicated”.
He’s not claiming it’s Schoenberg or anything!
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.