When a man gifted with charm seizes an opportunity, why do we say,“He’s in like Flynn”?

The Australian actor Errol Flynn had an amazing prowess with the ladies,
and of course the tabloids built this into a legend. During the Second
World War, servicemen coined the phrase “in like Flynn” either to brag
about their own conquests or to describe someone they envied. Flynn said
he hated the expression, but his own boast that he had spent between
twelve and fourteen thousand intimate nights ensured its survival.