You haven't watched enough of it, really watched it for technique, or read about it then. Seriously, I triple-dog-dare you to tell any 1st AC familiar with that production that it looks like an easy job. Flip a coin whether they laugh or want to punch you. You're looking with Joe Audience eyes. Great 1st ACs are the hardest working guy on the camera team and their work is practically invisible.
That's a 50mm and the stop varied between T1.4 and T2.8 and has many moments of "godly" focus pulling, considering there were practically no second takes. Regardless of all that, even in the short, chopped up version it's easy enough to see how impossible it would be for any dual-focus system. You can't move once much less constantly be adjusting.
The opening shot (from 1AM), before the first cut, has no point where the focus is static or at a fixed distance. Williams even stops moving forward while the camera continues on and goes from an MCU to a Medium. Then a bunch of cuts that are a second or two out of full-runs of the song (there is 24hrs of performance that went into the 4min posted edit BTW, and you can see it all), and we see the camera pulling back from a fixed subject. It's full of moments like that, and subjects going from back to foreground, through frame, end of song + subject handing off to new subject and restart of song.
The steadicam ops were wasted after walking backwards for 12-15Km a day but the 1st AC would be exhausted as well, not only because they were constantly working but from the stress of no second takes (they claim ~10 restarts out of 370 setups). No sir, it's the gymkhana of focus pull productions.
Watch harder.