I didn't get the whole "make a decision by smoking a cigarette. You're smart." Thing. Can ...

There was probably a note in that pack of cigarettes. Maybe a request to call the police to the harbour?

Also, the phrase you're referring to is a metaphor to not overthink things for too long - only for the duration of smoking one cigarette.

i’m not too sure either, i took it like how a cigarette goes out fast; implying that he made up his mind and was never going to leave mc in the first place type thing but idkkk