Agreed. Naruto btw. DID a bad job when Sasuke beat Deidara. It undermined the, up to that point, overwhelming aura pf Akatsuki. Before you had Kakuzu and Hidan beat in a tough battle, requiring many top heroes, one even dying. Sakura had the help of the person who creates puppets to defeat Sasori. It made kinda sense. And then Sasuke pwned an Akatsuki, made him even feel scared :/
Kimetsu now has very distorted "power levels". We can, ofx, speculate that this was the weakest of the 12, but the way he was beaten effortlessly was simply too crass imo. And he probably didnt even need his 11 breathing, he was superior in speed, strength alone.
Honestly, my favorable guess is that both these slayers are human-demon hybrids. Maybe drunk some demon blood, but kept it to a level thats not fatal. Would be a more satisfying explanation than "well. Duh, they just underwent harsher training"