That's not totally true. There was the Arrow 2.5 comic that filled in the gaps between Season 2 and 3 was written by Marc Guggenheim, who I think was show-runner at the time.
The Arrow: Dark Archer series was written by John Barrowman and he'd said at the time it was canon/he'd discussed the backstory with the show's producers.
And there was the Crisis on Infinite Earths tie in co-written by Guggenheim and confirmed to be canon at the time, focusing on characters whose actors' they hadn't been able to get back.
Didn’t that Crisis tie-in, which was in the Walmart books right, had the green lantern corps in it if I recall.
"It's fun and it's cool, so that's all that matters. It's what comics are for, Duh."
Words to live by.