Also the point is that Sukuna's normal dismantle output have a cap, he can't just raise them to his max output otherwise cleave would be literally useless.
It's the whole reason shrine uses cleave for anything with cursed energy and only uses dismantles on inanimate objetcts. From a number perspective for better wording:
Imagine 15F Sukuna's max dismantle output is 20, Ryu's durability is 25 so Ryu > Dismantles
Meanwhile cleave despite also being able to be a "10" when fighting someone who has "3" durability can be adjusted to be a "40" against Ryu so it cuts ryu in one fell swoop.
No one is denying that cleave
can be weaker than dismantle depending on who it's used. My point is Ryu's durability is so high that regular dismantles from 15F Sukuna can not cut through him so he has to resort to using cleave as cleave just adjusts to Ryu and cuts him regardless.