In Doctor Strange in the Multiverse of Madness (2022), Dr Strange meets Reed Richards of the Fantastic Four on 838 Earth:

Dr Strange: "Fantastic Four." Didn't you guys chart in the '60s?

Dr Strange is asking if the Fantastic Four had any popular songs or albums that were ranked on the music charts in the 1960s.

Why did Dr. Strange believe the Fantastic Four were related to popular music in the 1960s?