Didnt check the source code fully for a while.
1. hello-world example gives you a warning right into the face:
InboxSDK Developer Warning: Invalid AppId You've loaded the InboxSDK with an unregistered appId. Registration is free but necessary to load the SDK.
https://github.com/InboxSDK/hello-world
It would be great to see it truly open source without YOUR_APP_ID_HERE hustle
2. source code looks a bit old search /* @flow */
3. https://github.com/InboxSDK/InboxSDK/blob/main/package.json dev dependencies mixed with everything else
Overall it's still a very valuable project. Hacking into gmail or any other big SaaS service UI is multiyear unpleasant journey. Did it for LinkedIn myself. Lot of respect in InboxSDK team anyway.