Yes, I think the issue you pointed to is indeed the issue. I got a response back from Corona support on this issue:
http://docs.coronalabs.com/daily/plugin/google-iap-v3/migration.html
The parameter for the store.purchaseSubscription is just a string for the item’s id.
Again we do not officially support this functionally so any issues you run into may not be resolved.
I was able to confirm that the purchaseSubcription API does work! Just beware, per Corona, support for the API can’t be depended upon.