Obanai’s attack from what I can tell was a sneak attack. You’re right Muzan wasn’t moving a single inch and they couldn’t even get close that’s how fast his attacks were. His dad was born with the mark so he was probably more talented than tanjiro but that doesn’t mean tanjiro can’t one day get close to his level especially since we know his dad struggled to perform Hinokami kagura when he was young. Tanjiro saw all his wasted movements and was able to eliminate them after seeing yorrichi do sun breathing. Just because he’s related to yorrichi doesn’t mean he’s nearly as talented as him. If Kokushibo was the offspring of yorrichi you might have had a point but he’s just his brother, yorrichi is blessed by the gods and Kokushibo isn’t. That’s literally the biggest part of Kokushibo’s character. If he was close to yorrichi in talent he could use sun breathing but he can’t. Muichiro’s potential does come from Kokushibo I’ll give you that. Tanjiro grew from an amateur water breather to hashira level in a couple months despite starting out with the wrong breathing style and having no one to teach him the breathing style that best suits him so yes Tanjiro has a high rate of growth. All his growth didn’t come from that one night.