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.”