Is the statement "Allah is the only One who knows what is in hearts" incorrect, given that the angel and the devil also see what is in the heart by Allah's will?

The questioner was not mistaken in denying that anyone other than Allah knows what is in the hearts; this is part of the unseen, which Allah alone possesses. Allah Almighty may enable an angel or a devil to know something of this, but this does not negate Allah's exclusive knowledge of the unseen, because it is He who informed them or empowered them.