That article mentions that bundles are supposed to arrive in Gmail soon. My point still stands that Inbox features would be not-so-difficult to add to Gmail.

It's much easier than developing and maintaining 2 apps, and testing them on variety of platforms (Web, ios, android etc.) and device/browser combinations.

It was already getting confusing with smart compose (https://www.blog.google/products/gmail/subject-write-emails-...) coming to Gmail, but not to Inbox.