They're heads-up pros. Ivey and many other well-known names are not heads up specialists like these competitors. They'd likely have an edge over almost any "recognizable" name player.
Jason Les and Dong Kim both played in the previous competition, with Doug Polk (arguably the best heads up player in the world). They're reasonably in the discussion, and other factors such as interest and availability possibly factored in.
The $200k pool of money for an actual 'celebrated' poker player is absolutely not worth the time and effort. Hence why you get a few tiers down in terms of player skill. Still valid tests though, you can't only compare it to the top 1%
It's not a few tiers down in skill, it's a few tiers down in fame. Each of the four players named is likely a favorite over Ivey in this format. Being a specialist counts for a lot here.
“Your favorite poker player almost surely wouldn't agree to play any of these guys for high stakes, and would lose a lot of money if they did,” Galfond added. “Each of the four would beat me decisively.”
All-time winnings is not the best measurement as it doesn't take into account things like years played/measured, single individual large wins, etc. It's a decent list for one mode of inspection (winnings) but shouldn't automatically be taken as a ranking of players by skill.
Ivey is a very good player, for sure. In the context of HU play (as being discussed in the article), there are better.
That looks much more like he quit playing online. Saying he 'just got lucky a few times' is very odd. Ivey is considered one of the top pros by virtually everyone.
Beyond that, he (like many top pros), prioritizes lucrative cash games over tournament play (or exploiting an edge at baccarat, if the opportunity arises) which we can't track.