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.
Actually, I think Francis does believe it doesn't matter whether you're Lutheran or Catholic. And that belief is heretical. I'm just saying that in the particular sentence that was quoted, he was referring to how Satan sees all Christians--it doesn't matter to Satan whether you're Catholic or Lutheran... he hates you because you believe in Jesus Christ.
At other times, and even in other parts of that "chat" Francis has clearly deviated from the belief that the Catholic Church possesses the fullness of the Christian faith.
And you're right that his admission that the statements he was about to make would be considered heresy indicates that he did intend for them to be interpreted in an heretical way.