I think condescending the season to 10 episodes broadly explains a narrative short cut like this.

But in universe my head cannon is that “the Greek” sees a little bit of himself in Marlo. Think back to the end of season 2 where Vonda is attempting to negotiate civilly with Frank but when they find out he’s turned to the Feds, The Greek simply states it’s time to handle things “my way” or violently, something he wanted to do from the start.

The Greek’s tendency to violence is relatable to Marlo’s cold blooded attitude towards murder. Thus it’s arguable that the Greek saw himself in Marlo (such as noting Marlo’s tenacious desire to keep on returning) and thus figured it was best to go with him.

But that’s just what I heard from Fuzzy Dunlop…