Well, first of all, "The Incident" always happened in one way or another, even in the regular timeline. In fact, I'm not sure that the Nuke going off didn't always happen, even the "first time" around. Remember that when Jack and Sayid were first exploring the Swan Station they found an area totally blocked by tons and tons on concrete. Sayid even said that the only other time he had seen anything like that was at the nuclear incident site at Chrynoble(?). So that seems to indicate that there is a good chance that "whatever happened, happened" and that Jack was merely doing what he was destined to do (and had already done in the past before).
I assumed they "fixed" the hole they created by dumping the cement in. I seriously doubt they would have detonated a nuclear bomb, especially since 1) they didn't have one handy and 2) it was considered a last-ditch resort for stopping it should it be released again.
The fact that it resembled the measures they used to contain a nuclear meltdown is testimony to how powerful the island is more than a suggestion that it
was an actual nuclear meltdown. And if one nuke didn't stop it the first time, why would a second? Especially when no one would be there to fill it up with cement again.
So nah. I seriously doubt your assumption is right. The big reason is because the detonation
did apparently create an alternate universe where the island sank. The other universe is there to demonstrate what happened because of those actions and how irresponsible and selfish it was; the creative team has already stated this is an article.