Hello native English speakers.
Thank you very much in advance.

There are many contexts where native and non-native speakers are at an advantage if they understand not just polished, elegant language, but the full breadth of ordinary and unusual expressions (both those that are grammatical and those whose grammaticality is perhaps questionable). (original)

I guess :
if they understand not just polished, elegant language, but the full breadth of ordinary and unusual expressions =
1. if they understand the full breadth of ordinary and unusual expressions rather than just polished, elegant language.?

OR,

2. if they understand not only just polished, elegant language, but also the full breadth of ordinary and unusual expressions. ?

Which is correct by grammar and by the context of the original?

Question

Hello native English speakers.
Thank you very much in advance.

There are many contexts where native and non-native speakers are at an advantage if they understand not just polished, elegant language, but the full breadth of ordinary and unusual expressions (both those that are grammatical and those whose grammaticality is perhaps questionable). (original)

I guess :
if they understand not just polished, elegant language, but the full breadth of ordinary and unusual expressions  =  
1.  if they understand the full breadth of ordinary and unusual expressions rather than just polished, elegant language.?

OR,

2. if they understand not only just polished, elegant language, but also the full breadth of ordinary and unusual expressions.  ?

Which is correct by grammar and by the context of the original?

Answer

The second one is correct. The author is saying that skilled speakers understand fancy words, but they must also understand how average people use language everyday.

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