The Rogue archetype Arcane Trickster gains the feature Magical Ambush at level 9, which says
if you are hidden from a creature when you cast a spell on it, the creature has disadvantage on any saving throw it makes against the spell this turn.
Spellcasting > Targets explains:
A typical spell requires you to pick one or more targets to be affected by the spell’s magic. A spell’s description tells you whether the spell targets creatures, Objects, or a point of origin for an area of effect
The Area of Effect section adds:
Spells such as Burning Hands and Cone of Cold cover an area, allowing them to affect multiple creatures at once.
These wordings don't make it clear to me whether creatures affected by Area of Effect spells are also considered as having these casted upon them, and as a result the following question emerges:
Does Magical Ambush affect AoE spells?