There's a thin line between a boulder problem and a route.
In 1995, when the hardest grade ever climbed was still 8c+, cheeky Fred Rouhling announced he had climbed a 9b line in Les Eaux claires. Called Akira, the line followed the roof of a cave and finished through a very steep section of overhang.
Most Americans dismissed the grade. At that time, the strongest of them were struggling on 5.14b - roughly 8c? European climbers were stronger anyway. That same year, Swiss climber Elie Chevieux sent the first 8b+ on-sight. But even the Europeans weren't too inclined to believe Rouhling.
French man Jibé Tribout, who was competing with Ben Spoon and Gerry the Muppet for the unofficial title of world's top climber, was no particular friend of Rouhling. But he went on to try Akira. He said he found it to be a very dangerous line because "you hang at three or four meters off the ground and with some of the moves, you take the risk to land flat on your back". He thought it was a quick step from 8c+ to 9b, but reckoned that Akira was nevertheless much harder than anything that had been climbed before (read his interview in On the Edge n°102 if you can find it).
There was also an excellent article by Pete Ward and Tim Kemple in Climbing Magazine, featuring a discussion with Alexander Huber where he questions Rouhling's track record :
Huber gestures with his hand: “If Rouhling’s level is here,” he says, holding his hand at chest level, “and then with Akira it is here” — he holds his hand at his forehead — “then there should be many other routes around here.” The hand is level with his nose. “Where is this track record?” Huber asks. The hand moves to the side of his head, palm up. “Why hasn’t he done many other hard routes soon after Akira?”
Tim leans into the table and says, “Because he couldn’t climb for almost two years.”
“Why is this?” Huber asks.
“Because he had two kids, and his wife had brain surgery and almost died.”
Well since then, Rouhling has actually repeated a few other routes, including Fred Nicole's Bain de sang, 9a, though Dave Graham, who has also climbed it, thought it was easier than Wolfang Güllich's Action Directe (first established 9a). But Akira, on the other hand, hasn't been repeated ; neither by Huber, nor by anyone else.
What if Akira was a boulder problem?
What's interesting in all that debate is that everyone defines Akira as a route. But on the Youtube video, Rouhling uses no rope, no harness, no quickdraw. What if it was a boulder problem?
Its 9b grade would correspond more or less to an 8c+ in bouldering money. What else is available at that grade? Tonino 78 (Meschia, Italy, 15 moves, FA Mauro Calibani 2004), The Wheel of Life (Grampians, Australia, 60 moves, FA Koyamada 2004), Terremer (Hueco Tanks, USA, 11 moves, FA Fred Nicole 2006)?
The 60 moves of The Wheel of Life are actually considered to be a boulder line (the original grade being V16). What would make it harder than Akira then?