So Google is selectively oppressing certain app developers by using the "Rules for thee, not for me" ideology. Well, they've been doing it for years and that's why there's people in the US Government talking about anti-trust investigations... Hopefully something comes of it.
Gboard doesn't have in app purchases, the other one does. The rules are different because the apps are different.
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.