textContainers is an array element, maybe that's the problem. app.selection too is an array element, but maybe a different kind. I wouldn't be surprised if the problem has something to do with that -- not very helpful!
Did you try the textFrames from the story?
app.documents.exportPageItemsToSnippet( snippetFile , [id] );
where I get id from app.selection.parentStory.textContainers.id .
This is working with InDesign CC 2018.1 on Windows 10.
FWIW: I see the same issue as you do with exportFile().