I genuinely can't tell whether the article is suggesting that Inbox is for advanced users or for non-advanced users (and GMail for whatever the other user is). I believe, based on my own understanding of the two products, that GMail will continue to exist for advanced users, and Inbox will be simplified. But, the article is not at all clear on that; the last sentence seems to flip the meaning of the rest of the article and indicate that Inbox is the "advanced" one, while GMail will be made more simple. Or something.
It seems to me the other way around. I am a high volume user and I have relied on Gmail for almost 10 years. I signed up for inbox (because I am always looking for something better). I found it slowed me down. It was pretty but superficial. I prefer the plain but functional Gmail. Just one data point, I realize.
Thank you for clarifying. It makes the last sentence of the article make sense, but I think one would have to already know the difference between GMail and Inbox in order to understand the article with confidence.
Author can't English good.