I now find the source of Piper's calling Sproul not baptized. However, the idea to agree to disagree peacefully is what makes a good ministry among ministries.
Piper in his debate with Wayne Grudem on church membership; I'm sure I've heard of a sermon by Piper on Sproul's baptism but I have yet to find it:...
No, Wayne, this is not true. I would gladly admit Ligon Duncan or Sinclair Ferguson or R. C. Sproul or Philip Ryken to membership at Bethlehem (if I were allowed by our constitution), and in doing so I would not be giving up my view on the proper nature of baptism.
I would say to them: “Brothers, I think you are not baptized. But you believe on biblical grounds as you see them, with as much humility and openness to truth as God has given you, that you are baptized. Your understanding of baptism does not imply that Christ’s command may be neglected or that infant sprinkling is regenerating. You give good evidence of being born again and that you embrace Christ as your Savior and Lord and Treasure, and you manifest an authentic intention, on the basis of that faith, to follow Jesus as Lord and obey his teachings. Therefore, since there is good evidence that you are members of the Body of Christ, you may be members of this local expression of that body. But understand this: I will spend the rest of my ministry trying to persuade you that you and your children should follow through on the full obedience to Jesus and be baptized. In admitting you, I do not give up on my view of baptism. That is the whole point. We are finding a way to work on this disagreement from inside the body of Christ in its local expression.”
Source: Desiring God