I have often heard atheists question why God would bother to say this to Satan, as if He is acting out of pride, but God is rather fully aware of what will occur and provokes Satan into his sin in order that He may discipline Job and bring Job to deeper truths and understanding of God.
Job does not understand that God can use adversity for teaching/disciplining as well
Job sees God as some kind of full agent who can do as He pleases, with no real need to be held accountable to anyone else for He is above all and over all
I would venture to say that, at this point, Job has begun to sin against the Lord with speech, however, the thing that Satan believed would occur was that Job would reject the Lord, not just sin against Him.
It is important for ourselves to recognize this: that sin is not inherently rejection of the Lord, but rejection of the Lord is inherently sinful.
Whether or not Eliphaz realizes he has answered his prior rhetorical question is probably up for discussion. To me, it seems that he does not believe that God takes His full authority to the extent which the righteous also may suffer under His judgment.
Rather we know that God both has the right and the reason to do so, including to discipline his elect.