It sounds like her to be sure, but the story doesn't really hold up time or story-wise...
Alejandra is maybe 13 or 14 at most in the cinematic, Sombra is likely in her early to mid-20s. Morrison only became soldier 76 after the Omnic crisis; Sombra lost her parents in the Omnic crisis and Alejandra clearly has a mother in the cinematic and Morrison is clearly already Soldier 76.