Gaiden isn't a joke. What makes you think that way? I don't think a lot of people here are able to extract the point in this chapter. Kishi isn't out of his mind to downgrade Naruto and Sasuke like that, neither is he making a mistake or out of ideas in his writing, he knows exactly what he's doing. And yes, Naruto and Sasuke have been nerfed, and Kishi has mentioned it in the chapter as well. Firstly, the title of that chapter "An Unevolved Species Evolutionless?" is an indication of what's happening in this chapter i.e. the "nerf" that has been depicted. And next, the bald guy's words imply exactly the same thing. Kishi has mentioned the reason of Naruto and Sasuke's nerf through the balded guy's words that peace has made Naruto and Sasuke's battle skills rusty and lack of evolution due to peace is leading to their downgrade in terms of battle. Kurama also said the same thing about Naruto for a reason. That was the whole idea of this chapter.