John Piper is a Five Point Calvinist. From the limited atonement section of Bethlehem Baptist Church's doctrinal position "What We Believe About the Five Points of Calvinism" (compiled by Piper and "the Council of Elders"):
On the other hand we do not limit the power and effectiveness of the atonement. We simply say that in the cross God had in view the actual redemption of his children. And we affirm that when Christ died for these, he did not just create the opportunity for them to save themselves, but really purchased for them all that was necessary to get them saved, including the grace of regeneration and the gift of faith.
I tried googling "Canons of Dort" "John Piper" so far I haven't found anything relevant.
Anyway, when did you heard it? recently?
Ok, I think I found the article by googling "John Piper" Amyraldian. The author thinks that John sounds like holding onto amyraldian view.
Below is the statement of John Piper that the author highlighted
We do not deny that all men are the intended beneficiaries of the
cross in some sense. 1 Timothy 4:10 says that Christ is "the Savior of
all men, especially of those who believe." What we deny is that all
men are intended as the beneficiaries of the death of Christ in the
same way. All of God's mercy toward unbelievers -- from the rising sun
(Matthew 5:45) to the worldwide preaching of the gospel (John 3:16) --
is made possible because of the cross.
"Whence does this mercy flow to sinners? How is God just to withhold
judgment from sinners who deserve to be immediately cast into hell?
The answer is that Christ's death so clearly demonstrates God's just
abhorrence of sin that he is free to treat the world with mercy
without compromising his righteousness. In this sense Christ is the
savior of all men."
"But he is especially the Savior of those who believe. He did not die
for all men in the same sense. The intention of the death of Christ
for the children of God was that it purchase far more than the rising
sun and the opportunity to be saved. The death of Christ actually
saves from ALL evil those for whom Christ died "especially.""
"Therefore if Christ is the propitiation for all the sins of every
individual in the world, they cannot be punished, and must be saved.
But John does not believe in such universalism (John 5:29). Therefore
it is very unlikely that 1 John 2:2 teaches that Jesus is the
propitiation of every person in the world."
However, if one will go through the whole page the author referenced from, especially John's last statements
We can conclude this section with the following summary argument.
Which of these statements is true?
Christ died for some of the sins of all men.
Christ died for all the sins of some men.
Christ died for all the sins of all men.
No one says that the first is true, for then all would be lost because
of the sins that Christ did not die for. The only way to be saved from
sin is for Christ to cover it with his blood.
The third statement is what the Arminians would say. Christ died for
all the sins of all men. But then why are not all saved? They answer,
Because some do not believe. But is this unbelief not one of the sins
for which Christ died? If they say yes, then why is it not covered by
the blood of Jesus and all unbelievers saved? If they say no (unbelief
is not a sin that Christ has died for) then they must say that men can
be saved without having all their sins atoned for by Jesus, or they
must join us in affirming statement number two: Christ died for all
the sins of some men. That is, he died for the unbelief of the elect
so that God's punitive wrath is appeased toward them and his grace is
free to draw them irresistibly out of darkness into his marvelous
Apparently, these statements are typical of a Five-point Calvinist showing that statements 1 and 3 does not hold water.
The way I see it, John Piper made it clear that Christ died for the elect, differently from the non-elect, by purchasing actual salvation, unlike the non-elect, who receives common grace.
Lastly, it's easy to be confused by John's statement because Limited Atonement is usually defined in context of the Elect and the saving benefits of the cross. After all Mark Driscol used the term Limited/Unlimited Atonement and his brand of Calvinism was called 4.5 pointer. It pays well to understand what someone is really saying before we pass judgement.