Since the episode’s title refers to the comic book location’s name before it was shortened to “Star City” back in Season 4, it would make sense for part of the episode to be centered entirely on moments from years ago, before Oliver had to say goodbye to Felicity – Emily Bett Rickards exited the show at the end of last season – and before he knew about his secret half-sister.
The first part of Stephen Amell’s caption taps into the more expected explanation, that the mansion is part of a flashback sequence.