Блин, ну вот взял бы сам словарь да посмотрел. sure действительно
означает безопастность, хотя это значение и устарело. Но в контексте как
раз все правильно.
sure ( P ) Pronunciation Key (shr, shыr)
adj. sur·er, sur·est
Impossible to doubt or dispute; certain.
Not hesitating or wavering; firm: sure convictions.
Confident, as of something awaited or expected: sure of ultimate victory.
Bound to come about or happen; inevitable: sure defeat.
Having one's course directed; destined or bound: sure to succeed.
Certain not to miss or err; steady: a sure hand on the throttle.
Worthy of being trusted or depended on; reliable.
Free from or marked by freedom from doubt: sure of her friends.
Careful to do something: asked me to be sure to turn off the stove.
Obsolete. Free from harm or danger; safe.