@carcass
Can you provide the OE of this question. I have a difficulty understanding this sentence.

A quick google search turns up this interesting piece from Wikipedia.

"A scholar is erudite when instruction and reading followed by digestion and contemplation have effaced all rudeness that is to say smoothed away all raw, untrained incivility."

In the context of the sentence, it would make sense as to why talking slang would be a bad thing, especially during her thesis presentation.

The meaning of the sentence is this:

Nonetheless, his mentor asserted that she should write a thesis in standard English (or whatever is her language), she might not use slang and this indeed overrides years of learning.

Moreover, she has been attempting to being erudite, who has a LOT of knowledge such as a scholar, actually she miserably failed.

Ask for further assistance.

Regards

A slang by definition by good is a type of language consisting of words and phrases that are regarded as very informal, are more common in speech than writing, and are typically restricted to a particular context or group of people.
Basically it is restricted to a group of people. How can Erudite doesnt mean the same.
On the other hand Lucid means clear which can be interpreted as understandable to everyone.