Well in the article it clearly states that the age rating discrepancy between Gboard and Fleksy is due to the middle finger emoji.

1. Google didn't say that the reason for the rating was IAP and instead pointed to something that is common between that app and Gboard.

2. Other keyboard apps have IAP but still have an E rating: https://play.google.com/store/apps/details?id=com.gingersoft...

I find it very unlikely that Google is intentionally picking on this small app, but by being more lenient in reviewing its own apps, Google is abusing its position.