Nothing odd about that... DLC can not be exposed via this API, just connecting steam account is not enough to get DLC ownership data. Actually you need to have an online profile to scan and use an additional scraper on top of the api if you wanted to expose DLC owned. Only way you can do this, is if the user jumps through various hoops ;P
So this isn't just a problem with AI War, it affects all games with DLCs that are not a separate "game" entry.
To be more accurate, this is a problem with GoG Connect and it only doing step 1 of 2 required steps.