I don't think he compromises. Yes, he says that he welcomes "Catholics, Methodists, Mormons, Jews and ordained women" to study in his training programs. But if they come, they are going to hear the basic principles of the SBC. It hardly sounds as though he's compromising on the essentials of the faith. Reread the article.
"All his senior staff sign on to the SBC's doctrines, such as the literal and infallible Bible and exclusion of women as senior pastors. [...] Warren does draw absolute lines theologically. It's Jesus or hell. "Every human being is created by God, not everyone is a child of God," he writes in Life."
If he does, he didn't communicate it well in this article (or he was misquoted).
Even aside from whether or not Mormons, Jews, etc. are welcome in his training sessions, to indicate that he believes the line that divides to be "non-essential" is not a very theologically savvy thing to say (or fail to clarify).