The only place I can think where this pivotal memory MIGHT have surfaced would be if Strange encountered it while surfing through parallel dimensions, and one of the film’s villains — Dormammu, perhaps — unlocked the memory in an effort to distract the Sorcerer Supreme.
But yeah, unless you were up-to-date on Strange’s history, the sudden scene of a young girl drowning might have had you scratching your head in confusion.
Seeing as how there are no moments in the movie, either via flashback or triggered memory, where Stephen Strange is presented as a younger version of himself, a la Clark Kent in Kansas, Scott Derrickson likely is correct in that this scene wouldn’t fit anywhere in the movie.