Kaigaku didn’t know what it was
he was still vigilant, and thus being taken by surprise is no longer an argument
Head canon. He was confused.
calling it headcanon just is dictates an unwillingness to engage with a logical extrapolation and the premises supporting it, that isn't a valid counter to my conclusion nor my reasoning but an attempt at a deflection, using your prespective, your own interpretation of his confusion has as much basis- or if anything less- than my own about his vigilance towards the unkown tremor coming where he just watched a demon hunter fall.
At least from what i can tell, my own accounts for his experience natural deduction skill and wariness, yours just seem to look at the surface emotion rather than digging deaper into its causes and what would the appropriate reaction of the character would be given the situation at hand.
To reitirate, his confusion stemmed from the unkown tremor that made him vigilant around the area zenitsu fell in, even asking what was that with an edge in his voice and clear unease, and even ignoring this, his sens of sight was in line with zenitsu's own path towards him- assuming from there zenitsu, for some reason, only becamse visible as he was 5 meter away from him- kaigaku still failed to meaingfully within that distance, not even managing to bring his arm up or twist his torso or even move at all, indicating a speed difference reaching an orders of magnitudes or more.
And he wasn't even confused,
noting that zenitsu still has strenght in the manga and being quite vigilant of him in the anime showcased him not moving a muscle.
Indicating zenitsu pretty handily statued him.
Kaigaku is overconfident. It is likely he thought Zenitsu couldn’t cut his neck so he didn’t move.
This is more headcanon than my interpretation could ever be...
First of all, where did you get the notion that he is overconfident from?
From what i remember he is a cowardly and insecure character deseperate for acknowldgement, but besides that, you're just assuming your own conclusion here, based On what did you infer kaigaku was
1 Confident in that instance
2 Was able to not only see and register in zenitsu's incoming attack, but also rationally and calmly conclude the attack wouldn't harm him
3 Decide to then stand still, not move a muscle, and just accept it fully beleiving in his own durabilty
you're skipping the conclusion forming process and jumping straight to it, then go back from that assumed conclusion to defend it, this is a flawed methodology of arguing.
but well...regardless of that,
you're just straight up wrong lol.