Spalk
Kakashi doesn’t use seals or weights to train his body. The only weight training he’s been shown to use in canon is cliff climbing and his challenges with Gai. It looks like this fic is going to go with the forced plot of Kakashi always being stronger than the MC no matter what. In fics, I’ve seen even when the MC has Rinnegan and every bloodline imaginable, Kakashi will still beat the MC in their first assessment for the genin team. The MC could literally be Madara reborn into Sasuke’s body, and the author will still have the MC lose. I don’t think Kakashi could copy that technique of Sasuke’s without using his Sharingan. He’s not that much of a genius. Remember, he couldn’t even copy the Rasengan that easily and accidentally created the Chidori because he couldn’t get it right. He’s not know to replicate jutsu without using his Sharingan, so I definitely think this feat is forced plot.