"Thought it not" means that Jesus Christ never thought it, not once, not ever.
What did Jesus never think?
Jesus never thought "it robbery to be equal with God."
That is to say, that Jesus didn't think like Satan, and his rebel angels thought. They thought they could steal the authority of God and by taking his throne.
Jesus never thought he was equal with God, his Father.
But some of you think you can authorize yourselves as equal to the task of editing God's written word. Of course in that pursuit the same pretense God's written word doesn't exist on earth anymore.
The niv is stating that Jesus was equal to God, his Father, which can't be.
What matters is not how well you claim you can translate, which you absolutely can't honestly do. But rather, can you tell the truth about Jesus Christ, confess your lies about him, and repent?