What's really interesting between Stu's era and today (or even the last 20 years) is technology.
Live poker is a pretty slow game. You might get 30 hands in per hour and play with 5-15 other people (rotating in / out) during a few hour time span. The bottleneck is mainly the physical action of moving cards around, counting chips and slow players due to the environment.
But online is a totally different game. You could pretty easily play 4 tables in parallel by tiling them on your monitor. Then there was software you could run that overlays stats about every player you encounter (collected automatically). You gathered tremendous amounts of data that you could then analyze in real time to help make decisions and then also look at a hand by hand audit with those stats to help evaluate your game after the fact.
Some people ended up playing 10+ tables at once (even at pretty high stakes), and an online game can easily get 90 hands per hour just on a single table. So if you do the math, you could be cranking through 900 hands an hour vs 30 in a live scenario. That's 30x more hands played per hour. If you factor in playing for tens of thousands of hours, the amount of experience you can gain online in such a short amount of time is crazy. You could put in many millions of hands online vs low hundreds of thousands live, and then have a ton of data to help figure out the game better.
1 year of playing online like that gives you 30 years worth of experience. Of course it's not as high quality of experience since 10 tabling is tough, but even if you dropped it to 5 tables, that's still condensing 15 years worth of live play in 1 real life year online. So it's no surprise that players now have a huge upper hand in being able to improve their game.