That's not the same sword Sasuke had before. Sasuke's old sword was a Kusanagi Blade which was said to be able to cut/pierce anything.
As for how they got beat, it doesn't really matter how rusty they are, to be honest. Sasuke shouldn't be so rusty that he can't activate a Susano'o ribcage, and it's not like Kurama gets weaker over time, so Naruto's durability with a cloak should be able to hold up against a sword.
It really just all boils down to how Kishimoto wants things to play out, the amount of sense certain things make can be discussed but ultimately none of it matters because we don't write this.