Megillah 18a states
אִילֵּימָא דִּכְתִיבָה מִקְרָא וְקָרֵי לַהּ תַּרְגּוּם, הַיְינוּ עַל פֶּה
If we say that the Megilla was written in the original biblical text, i.e., in Hebrew, and he read it in Aramaic translation, then this is the same as reading it by heart
Why is translating on the fly the same as reading it by heart?!
When you read by heart you are relying on your memory; here you are relying on your knowledge of two languages which seems to be very different (from the cog sci POV).