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

First he implied that he's gonna come back. And I agree with ChezSoi, there probably was a note in the pack, maybe he meant to wait for the duration of a cigarette to call the cops or something. It's probably gonna be cleared up