Those rules only pertain to NPCs and the levels beyond 10 are there to make the calculations for high-level encounters work. None of that is meant for PCs because as Sam mentioned, OL is designed to be played up to level 10 PCs, from there on all bets are off in terms of balance.
From the description of the boon: “You peer into the future to gain insight into a course of action, an event, a person, or a place. Examples of this boon include a fortune teller…”
So I think that should make the intention of the boon a bit clearer.
Let’s get to your current question:
3 rounds should probably be the median for most combats, at least by the basic design of the system.
Statistically, that’s overkill, so I wouldn’t worry too much about it. Let the player have that and instead design encounters that turn their strengths into problems that need to be solved. So, for example, create enemies through which they can shred easily but whenever they strike or destroy the opponent, they face consequences. May the opponent be an acidic blob that spills acid all over the place when struck or a clockwork that blows up at the finishing blow, create encounters that demand the players to adapt instead of throwing higher numbers at them.
At high-levels you need to become more creative with your encounter design and dig deeper into the bag of tricks to challenge and surprise them, so I would advise to running fewer encounters per session on average, but with more bells and whistles, to keep them on their toes.