In relation to your code the issue is you are not returning the data from the fetchData() so that it can be used by the createArticle(). This value of $articleData is empty at this point.
// Run the createArticle bulk data import to articles function
Update your code with
// Fetch the JSON data
$articleData = $this->...
I found when running CLI scripts that other things that you don't really need to execute, i.e. plugins, will cause failures because they don't check themselves to disable if they are running under a CLI environment and so they call things that aren't available to you and fail your script. To 'ignore' these errors I add several lines to bottom of my class to ...