Amell was talking about how important the Black Canary is to Green Arrow lore, and said it wasn't a hallucination, but I'm not sure if it was even hinted at (beyond our assumptions) that it is the exact same Laurel that we lost last year. It seemed vague enough.
If it IS Black Siren, it could stlil completely work.... originally employed by Prometheus, but totally swooning to Ollie's side once she finds out he is alive - if on her Earth, Ollie died at sea, it could have been the trigger for her going darkside. This could be the reverse Artemis; Siren's redemption story, AND the fixing of the BC/Dinah character, as I suggested both last year and after the anniversary episodes stressing of the Laurel/Oliver love story.
I assumed hallucination after seeing this week's Legends of Tomorrow.