For what purpose does tan juxtapose "i am not a scholar of english or literature" with "i am a writer"

The best answer for this question would be:

It is because she is creating a paradox between her as a writer.

This means that the audience were expecting something of a speech of a scholar, which people have high regards about. She tells people that she is not a scholar, in order to lower down the pressure that they have on her, and not to criticize her based on the grammar or word choice. Tan openly tells the audience that she is a writer, because a scholar and a writer have their differences, scholars study the language that they are using while writers freely express their thoughts through words.