If you got rid of the second part of that sentence, it would just be...
> You engineers do not understand the user
And they would be correct. From what the article said, it seemed the Gmail team (not just designers, the whole team) had mountains of data defending their choices. Their data proved that Gmail was becoming harder and harder for regular people to use, while engineers continued to ask for more advanced features. The only real solution was to split the interface between advanced users and normal users. As hard as it is to believe, especially in Silicon Valley, most of the people using Gmail are not engineers and do not work at Google. So while I'm not claiming that designers have all the answers, they can at least prove that this needed to be done. I would agree with them, I think most people who are going to use the web interface of Gmail are casual users and the UI changes work well for them.
Given Google's immense size, I'm very glad they're taking this approach...attempting to satisfy both the casual emailer and the power user and still providing a great service behind the scenes.