The particular instruction " Use the Symbol as the Code for each currency" is put here in the context of importing Country+local+currency.csv in the 'currency list'. Instead of manually entering local currencies as list members, you can use an import action wherein map 'Symbol' column to 'Code' field in import mapping action as shown below,
Once the currency list is imported with local currencies and symbols as code, Go to SYS03 Country module
-> add line item Local currency formatted with currency list. As pointed by you, one can either manually map local currencies to countries or import it in the below manner,
1. Go to grid view-> Data-> Import country+Local+Currency.csv file and the follow the highlighted mapping
2. G2 country tab shall automatically get mapped-> SYS03 Country Details Line Items tab should have the follow mapping,
Run the import. This will import local currencies rightly mapped to countries without any manual intervention.