Annesley's explanation is absolute garbage. It sounds like he was making it up as he goes.
He also never said that it was the "correct call", but that he is "comfortable with it" and that "it's not black and white."
The NRL's rules clearly state that only a referee's decision that results in a structured restart of play can be challenged. He didn't make a decision that resulted in a structured restart of play.
It's written in bold in the laws of the game
"2. A challenge is only permissible in instances where the referee makes a decision that results in the match recommencing with a structured restart"
https://www.nrl.com/siteassets/operations/documentation/nrl_laws_interpretations_2020.pdf on page 22
"“The rules around a captain’s challenge allow for a challenge of any decision made by a referee to stop a game,” Annesley said."
This is a flat out lie and a direct contradiction of the written laws of the NRL. The laws of the game clearly state that a challenge is only permissible where a referee makes a decision that results in the match recommencing with a structured restart.
The referee had made no decision that had resulted in a structured restart. Therefore a challenge is not permissible.
And what decision did the ref make which caused him to blow the whistle once to stop the game? He didn't make any decisions at all. He blew the whistle once to stop the game, for reasons unknown.