No, he's not. He's stating that Satan sees all Christians as one. And he's right, at least to a large degree. Satan doesn't care whether you're Lutheran or Catholic. Either way, you profess that Jesus Christ is Lord, and for that he hates you and wants to destroy you.
Catholic teaching is that unity is only found in the Catholic Church. It has never taught that various denominations "are one".
That's true. But Lutherans are a whole lot closer to Catholic than Hindus or Muslims. Close enough to get ol' Beelzebub pretty ticked off. The pope, at least in this statement, did not say that all Christians are one. I'm no great fan of this pope, but that's not what he said here.
But there is someone who knows that despite our differences we are one
Does Francis disagree here? Or is he stating that the Devil recognizes that "we are one"? Considering Francis' comments about his so-called "ecumenism of blood" weeks ago, he is absolutely stating that we are one. And he knows it sounds heretical .... and quite frankly doesn't care.