More is known about optimal fuelling, hydration and sleep. Improve those and you improve your daily training. Better quality training compounds and allows you to reach closer to your talent ceiling.

Kerr also had a system set up so his bedroom had less oxygen than the rest of his house (to mimic sleeping at altitude).

He had two pacers breaking the air for the first 1,000m (although he had to do it himself the rest of the way, which was bloody impressive). Meant he could relax mentally for the first 2.5 laps and didn't have to focus on pace. I think El Guerrouj set the previous WR in a race without pacers.

They also had pacing lights on the track which helps the pacers run at an even pace.

And there are all sorts of innovations like taking sodium bicarbonate to reduce muscle acidity, nitrates to dilate the veins and increase blood flow to muscles and high doses of caffeine to reduce the rate of perceived exertion.

As someone else mentioned, track surfaces are generally a little bouncier now than they used to be.

Surely raw human potential cannot have progressed very much at all in the (at most) two generations represented by the 27 years the record stood.

Granted, the population is significantly higher, so it is more statistically likely that we've produced a genuinely faster human than existed 27 years ago.

I think it's fairly well accepted that most of the records being broken now are down to tech, nutrition, and aids. Springier shoes, mechanical pacers, better 'fuels', deeper understanding of exercise periodization, etc.

Give the old record runner all of the same boosts, the same training, I can't imagine he'd be noticeably slower, perhaps within hundreths, but I'd bet within a tenth or two.