it's Actually Very easy Check the Video
He said Inuarashi and Nekomamushi can't be traitors because they proved loyalty. So did Kanjuro.
If you read the spoilers, the reason why Kinemon couldn't believe him initially was because he had proven loyalty.
The person who made that video got it right, granted, their reasoning is not correct. In essence, he made a good guess.
There was no evidence or much hint.
He said Kanjuro telling Shinobu to calm down = His wanting Law to get cut? Looks more like he's actually trying to calm her down. That's what an ally does. Calms people down and stops them from acting irrational.
He says Hawkins using Kairoseki nails against Law is proof Kanjuro was the culprit. Hawkins fights with nails in general. Why wouldn't he have Kairoseki nails to fight DF users to begin with?
His reasoning is incorrect, there really was no hint or evidence.
It was literally Oda choosing to making Kanjuro the culprit which is why he had to make up new information that we didn't know about to explain why he was the culprit.
If Oda didn't say:
- He was a Kurozumi.
- An actor.
- Wanted to die.
We'd at this point have no evidence as to why he's the culprit or likely the culprit.