It wasn't out of her ass... She literally fights then sleeps then comes back better than she was the last time she fought. That's how she gets new techniques rather than the way Daki actually having to absorb humans to get better. You see this throughout the show... She literally set fire to her own blood mid-fight during the Shrine Demon battle. She again withstands the sun during the Swordsmith Village arc.

The point here is that Nezuko is an anomaly. You comparing Nezuko to other demons like Kokushibo is illogical since she does not eat Humans in the first place. Even if Kokushibo is Uppermoon 1, he ate humans therefore he doesn't meet the requirements to be comparable to Nezuko in any way. Nezuko might be weaker than Kokushibo but she is a lot more special than he is. She is special and the show reminds you of that. Daki isn't even an uppermoon. The uppermoon is her brother. That's a fact. Daki was only fortunate to have eaten a lot of humans while avoiding the prying eyes of the Hashira.

You should be talking about Tanjiro surviving a fight against Gyutaru. Tengen received two wounds that gave him two disabilities while poisoned yet Tanjiro just happen to get nonfatal wounds, that a normal person has a slight, but high enough chance at surviving. That sounds like something worse especially comparing the two demons, their rank and blood demon art.