"Sofia is pouring tea into the cup."
Translation:Sofia verŝas teon en la tason.
No, indirect objects are marked with the preposition al.
"en la tason" is a prepositional expression for a direction (into the cup).
Some prepositions take the accusative to distinguish location and direction: en la taso (location: in the cup) – en la tason (direction: into the cup).
In German: in der Tasse – in die Tasse
In Russian: в чашкуе– в чашку