In the defence of the creator, I assume they're goal was not to create a "newsletter manager", they probably just saw that as low hanging fruit to relieve some of the pain of managing an inbox.

I can easily see them adding more and more features which improve email and nudge users to inbox zero.

I think this is a decent starting point, and for a repo that is only 5 months old, it looks like they've done a decent job of understanding a good UX, and showing the user the emails they are most likely going to want to archive, etc.