Because he is. Amaterasu being sensed is already shown more than once, Sasuke's arrows shown to be sensed and dodged by SM Kabuto. Naruto shown to sense and dodge things faster than him as he did to the 3rd Raikage who Naruto compared to A in speed. You do know Naruto literally have to do nothing to Susanoo right? Sasuke stated that Susanoo aches every cell in his body so the long it's up the more pain Sasuke is doing to himself and since Susanoo takes both eyes it makes Sasuke blind faster as well. Plus he's using up his chakra while Naruto can use his clones to surround Sasuke, use Rasengan on the ground to cover up their movements as he did with Nagato and the Rinnegan.
Frog song takes Susanoo down and paralyze Sasuke mind and body. Kabuto's sound genjutsu took down Itachi and Sasuke Susanoo and paralyze their bodies, frog song is stronger as it paralyze the mind as well. Sasuke have no knowledge about frog song, the toad Sage or that Naruto's frog kata allows him to extend natural energy around his body to hit his target and Sasuke can't sense or see Natural energy.
Oh and you do know both Pein and Naruto fought each other not at full power? You know the Pein part obviously with him fighting Konoha prior to Naruto but Naruto was training before returning to Konoha and he made 2 clones before being summoned back to Konoha so a Naruto was at 1/3rd the power. He sacrificed power for time pretty much and he still beat 5 Peins with SM then after Minato closed his seal he fought in SM and base with Deva path.
Naruto knows about Amaterasu, he seen it in part 1 and when they went to where Sasuke fought Itachi. Naruto has the advantage in abilities and intel.
