Well, he would be wrong. The KJB does not contradict itself, but the RSV does. The KJB says Jesus said "I go not up YET to this feast". Jesus did not say he was not going to the feast, he simply said he was not going yet.
But he would be correct using the RSV. The RSV says Jesus said he was not going to this feast, and then two verses later goes. And notice he quotes the RSV.
1. He quotes both. In both cases he sees it as Christ lying. And from a skeptics point of view, he is correct regardless of the version. Even the KJV rendering to a skeptic would make it appear that Christ is lying. So, to a skeptic, it doesn't really matter which version you present since he will tend to find fault with both.
2. And even if the skeptic grants that "yet" negates any contradiction between vs 7 and 10, he would still be reasonable in claiming that it represents and equivocation on Christ's part and, and such, may not represent Christ lying but does represent Him as being deceptive. And, if one looks only at those 3 verses, then the skeptic has a valid point.
3. However, if one looks at the larger context, one realizes that the supposed contradiction in the RSV and the supposed deception in the KJV are the result only of insisting on looking only at those 2 verses. Certainly in either the KJV and RSV rendering there appear to be some difficulties when one looks only at those two verses. Look at the larger context however and we see that Christ's statement was about going or not, but about going publicly or not.
Read the preceding verses in the chapter and we see that the topic is not about going to the feast per se, but about going to the feast publicly. His disciples urge Him to go and show the miracles to his followers, to go and make known to the world that He is the Messiah. Christ's response is that it is not yet His time - not that it wasn't His time to go to a feast, but that it wasn't His time to go to such an event publicly demonstrating the fact that He was the Messiah.
So, in context, His statement in vs. 8 is not that He isn't going at all, but that He isn't going publicly. If anything, the RSV makes this more clear than the KJV. The "yet" in the KJV makes it seem that His going or not is at issue when in fact the context make very clear that the issue is whether He will go publicly or not.
Contrary to the KJVO logic, this "yet" should not be seen at a clarification that He is just waiting till later in the day/week to go. Instead, this "yet" should be seen as referring to His coming publicly to Jerusalem at a later date - that is, when His death was drawing near
4. What really bugs me about the your argument above is that it grants the error that the skeptic makes in the first place - that one can look only at vs 8-10 to understand what is really going on. The proper response is NOT that the KJV renders it better, but that the skeptic was wrong in the first place to ignore the larger context. Just as the skeptic is wrong in ignoring context to show fault with Scripture, so are you wrong in ignoring context to show fault with the RSV. Take the proper approach - look at the context - and any difficulty vs. 8-10 presents for the RSV, much less for the Bible, disappears.