Daniel has the theory of variables, but how is Jack a variable? What choice did he have that he could change? We know he always tried to detonate the bomb.
In fact, if him detonating the bomb has changed the timeline, then what happened in the original timeline, they all stayed at the Barracks doing nothing? No, they always tried to detonate the bomb. We know this because we see the effect of their actions in the original timeline. Daniel was the one who started the idea of blowing up the bomb, and we can clearly see at least two (possibly three) effects on the original timeline of his actions about this in the seventies:
After he tells everyone about the bomb detonating theory he goes with Jack and Kate to the Others' camp and Eloise shoots him. This always happens because Eloise remembers in the original timeline that she shot him, meaning that he always went to their camp in search of the bomb as a part of the time loop. Daniel probably realizes as he dies, that his death always happened and that his mother knew, proving that he didn't change anything.
Before this, Daniel tells Chang that he has to evacuate everyone, which later on, other Losties confirm. As a result of this, Miles and Charlotte are sent into the real world which always happened as a part of the time loop as they would not be in the freighter mission.
Although this is not quite clear, Richard Alpert says, in the original timeline in 2007, that he saw the Dharma Losties dying, which could mean that he just met them when they went to extract the bomb and assumed they died after the blast. This, once again, could mean that Daniel coming to the camp and Jack and Sayid carrying off the bomb always happened, since Richard Alpert remembers it, but it's not all clear in his wording.
This seems to indicate that Daniel always came to the Island and always told the Losties about the detonation, prompting Jack to try to detonate the bomb, ultimately resulting in Daniel's death, and possibly, Richard Alpert's believing that they died.
So it seems established that Losties always tried to detonate the bomb at the Swan. However, some say that Jack was not the variable Daniel was talking about, but that it was Juliet.
Some say that the bomb not exploding and everything up to that was indeed all part of the original timeline, and has always happened, but that Juliet changed everything by detonating it. How could this be? There's no 11% chance of something happening. Sayid always shot Ben, Hurley and Jin always saved Jack and Sayid from the shootout with a van, Chang's hand was always crushed etc. This implies that Juliet was always entangled in the chains, and always fell into the pit, as a part of the original timeline. And of course she detonated the bomb in the original timeline: if she just risked a perfect life in the seventies because she wanted to blow up the bomb, fell into the pit, barely staying alive for enough time, but clearly dying, and saw the bomb that she wanted to detonate, had a chance to blow it up, would she really pass on it?
This means that detonation of the bomb is a part of the original, and only timeline, and that they didn't change anything in Season 6, but they probably did time travel.This is also in tune with every other episode of Season 5 prior to this, where everything the Losties do proves to be a part of the time loop they're in and everything that happened happened, although the theory itself is kind of stupid and useless in the fifties and seventies, since the Losties don't know what actually happened in the first place.
The only event that is dubious to the WHH theory is Daniel meeting Desmond; for all we know, this could be a part of the original time loop, and Desmond remembering it when he needed to could be the result of being brainscrewed by the time travel and "special", or could be indeed changing the timeline, which still wouldn't mean WHH wasn't right, since Desmond is "special", and, as Daniel says, the rules don't apply to him. So, he's a Joker when it comes to these theories.
However, further proof of WHH theory of unchangeable time loop is Eloise Hawking's comment that if the Oceanic 6 doesn't return to the island, "then God help us all". This could be because she knows that her young 1977 self meets the Losties from 2008, and that if they don't go, the meeting wouldn't happen, causing a time paradox of unknown consequences. Of course, she could just be guessing, but if she does know what she's talking about, this would imply that changing anything in the original timeline, and destroying the loop, would cause a paradox.