Its never been proven canonly that Skynet has been defeated in any of the incarnations. The only time that you could potentially claim that it was defeated was in "The Terminator", but that timeline was overwritten by the subsequent films.
Yes, Kyle flat out said that Conner beat Skynet and it sent Arnold Part 1 back to 1984 as a last resort.
But they also showed a flashback/flash forward in Part 3, of John celebrating with his troops, with his victory over Skynet.
That's farr more proof and evidence then Skynet being the ultimate winner. The only victory Skynet ever got was Judgment Day. And going by Part 3's established canon, you can only hold off the pre-determined you can't completely change it. So Conner will always win. No matter what.
Conner is pre-destined to defeat Skynet. No matter what time line. Always was. Always will be. It's his character arc.