This would be tricky for most studios making comic book movies to pull off, but the reality is that Marvel Studios currently has four different projects that are currently being made. Doctor Strange, and Guardians of the Galaxy Vol. 2 have both completed production and are now in the editing phase, while both Spider-Man: Homecoming and Thor: Ragnarok are still currently filming. It's even possible that the filming session had Stan Lee doing what he needed to do for scenes in The Avengers: Infinity War and The Avengers 4, given that both of those movies are now in pre-production down in Atlanta.