Even Matt M from TV Line wondered about LL/KC's placement in the poster when it came out:
"But it’s the staging of the next tier of costars that seemingly ranks Ollie’s gruff island buddy Slade Wilson (new series regular Manu Bennett) higher than his lithesome lady love Laurel (original cast member Katie Cassidy)."
Like you said, they were leading completely different lives and were always hiding something from each other. That all we were ever shown of Oliver and Laurel's relationship, whether it was before the island when he treated her like crap (cheated and lied), or post-island, when he sidelined her and lied to her about who he was and what he was doing. Yet the characters were made to say stuff about how they were meant to be together (Helena, Tommy), how Oliver was better around Laurel (Moira), how Laurel knew Oliver "in her bones" (Oliver, Laurel). The Oliver/Laurel relationship was the worst case of "tell, not show" I've ever seen on TV. I think that SA, who had shown how much thought he put into the character and the show in the early seasons, saw that disconnect and played it out in a way that made sense to him, which was Oliver resolving his guilt for how he mistreated her and then moving on.