How to import symbols from a symbol library?

I need to import symbols from an Ai file to other documents by script, then place them by replacing objects.

I have no problem with the replacement of a symbol when it's part of the symbols of the document.

What I fail to do, it's the importation of a symbol collection from an Ai file to the current document.

I load the symbol document as a symbol library, in this way:

var openOptions = new OpenOptions();
openOptions.openAs = LibraryType.SYMBOLS;
var symbolDocument = app.open(File('/d/test/symbol_collection.ai'), null, openOptions);

It works well as it opens a symbol collection window in Illustrator.

But I don't know how to get these symbols to copy them into the current document.

When I invoke symbolDocument.symbols, I get a 'The document is no longer open' error.

So, how to copy symbols? How to access this symbol collection loaded in Illustrator other than using the symbolDocument?

Thanks a lot.