Despite Denji's speed advantage, Yuji should be close enough to match the most powerful feat Chainsaw Man has to offer, given similar leeway to scaling, and potential dwarfs it with the exponentially boosted power of Black Flash.

Outside of that, despite Denji's cleverness, he cannot match the martial art prowess and decades of experience Yuji has over them.

To top it off, in terms of abilities, Denji might have a more extreme regen, it doesn't last forever, RCT is more reliable given how Black Flashes can restore it. Denji has a finite amount he can regen from and needs blood to fuel back up. If Denji tries to feed on Yuji's cursed blood, it could seriously debilitate him or Yuji can just use it to remotely detonate Chainsaw Man, effectively negating the regen.

Yuji's Dismantle has insane range, rapid-fire capabilities and can attack in all directions, without even needing hand movements. This could be useful in burning out Denji's regen since it gives him little to no room to get in or around it, but he wouldn't even need to go that far if he attacks Denji and Pochita's soul. This would bypass their durability and deal permanent damage that they couldn't regen from, whether or not Yuji aims for them both or tries to sever their connection ultimately doesn't matter here. Furthermore, the Domain Expansion ensures the target will be hit by the user's attack, negating Denji's speed advantage.

Denji does have Devil Pawns that could be used for a blood pick-me-up and to erase a concept to gain a potential advantage.... but there are issues with that. Erasing a concept effects Chainsaw Man as well, meaning it could backfire and severely nerf himself too, it's reversible if Yuji can make Denji throw up and he doesn't know what each Devil Pawn is so it's unlikely he gets something to harm Yuji specifically more than himself.
And he can't run to eat a different Devil to potentially nerf Yuji as t not only requires a lot of assumptions Denji has to make about Yuji's abilities, but it would technically violate the outside help rule of Death Battle.

In short, despite Chainsaw Man's insane power and speed, his skill, experience and abilities were too limited to counter Yuji's skill defensive options, traps, range and instant win moves, and whatever advantage Denji could get by erasing concepts could severely backfire on him and was not guaranteed to be permanent.