The biggest crock of bullshit wasn't anything the demon slayers were doing. It was that Rui somehow managed cut off his own head first.

- He shouldn't have known that Nezuko was going to attack him, or could.
- Both Nezuko and Tanjiro were burning away his threads like crazy.
- The attacks and the power behind them took him completely by surprise.

So how did he manage to both form new threads amidst all that fire consuming them, near his head when his arms were well below his neck, and do it despite the fact that Tanjiro's demon slaying blade was already halfway in his neck?

Plot armor, that's how. For a shitty, obnoxious villain character that had long overstayed his welcome.