The caption of his tweet makes it explicitly clear that Flashpoint and flashbacks have nothing to do with this episode.
Another option is that Sheridan is playing the Earth-2 version of Robert, who is still alive and is actually that universe’s Green Arrow.
The clear answer might have been that Robert returns through a flashback or that the universe rebooting Flashpoint makes it so that the character never died at all, but Guggenheim is one step ahead of our speculation.
It’s possible that Robert was resurrected at some point after his suicide, perhaps through a Lazarus Pit or some magical properties, though that feels like well-trodden ground at this point.