Arrow Season 6 could totally pick up months after that presumably deadly event to show us what happened in the days immediately following, and that would still technically be considered a flashback, without the show needing to specifically hit up The History of Oliver.
Exactly how far that Arrow flashback will take us remains to be seen.
The finale ended on the dramatic conclusion of Lian Yu literally exploding, so I can’t imagine the first episode of Season 6 picks up in the immediate aftermath, with Oliver trying to search for survivors with William by his side.