But what they concealed before has [now] appeared to them. And even if they were returned, they would return to that which they were forbidden; and indeed, they are liars.
Wishes and Hopes Do Not Help One When He Sees the Torment
Allah mentions the condition of the disbelievers when they are made to stand before the Fire on the Day of Resurrection and witness its chains and restraints, along with seeing the horrible, momentous conditions in the Fire with their own eyes. This is when the disbelievers will say,
("Would that we were but sent back (to the world)! Then we would not deny the Ayat of our Lord, and we would be of the believers!") They wish that they would be sent back to the life of the world so that they could perform righteous deeds, refrain from disbelieving in the Ayat of their Lord and be among the believers. Allah said,
In the second verse (28), Almighty Allah, who is All-Knowing and All-Aware, exposes the reality of their wish, which would be nothing but the compulsion of circumstances, by making it clear that these people have always been addicted to lying and that they were liars even in their present wish under trial. The root of the problem with them was that they did not relish the realities brought before them by the prophets of Allah. They knew that the prophets were true, yet they raised dust to keep such realities unmanifested either because of their chronic obstinacy or worldly greed. But, in the Hereafter, every-thing came out in the open. They saw the power of Allah with their own eyes. They realized that the prophets had told the truth. Even, the life after death, something they had denied all along was before them, really and physically. Then, they became direct witnesses to reward and punishment and Hell. After that, they were left with no argument to continue their attitude of denial and opposition. So, when they wished to be returned to the mortal world, that was mere absence of choice.
Then, their All-Knowing, All-Aware Master said that this was what they were saying for now, but if they were to be sent back into the mortal world once again, they would simply forget all about their promises and resolutions and will revert to doing what they have been doing before, and would start getting involved once again with the unlawful from which they were stopped. Therefore, what they were saying now was also a lie and a ploy for deception.
That their present statement has been called a lie could also be based on their ultimate deed, that is, their promising not to lie if returned to the world was not true, for this would never happen because they would still do nothing but keep lying when they go there. This lie could also mean that whatever they were saying at that time was not prompted by true intention. They were doing so simply to buy time and avoid punishment while in the heart of their hearts, they still have no intention of doing it.
Tazkirul Quran
Maulana Wahiduddin KhanOn ayahs 28–29
For people who have this temperament, all moralizing is in vain, because in this world of trial there is no such rule as prevents them from fabricating counter arguments. Even if they do not manage to concoct such arguments, they will contemptuously ignore the truth, saying, ‘What is new in this? This is the same old story which we have been hearing for such a long time.’ In this way, even after being convinced of the veracity of moral precepts, they will find some excuse to reject them. Before God such people are doubly guilty, because not only do they themselves refrain from admitting the Truth, but they also make the Divine argument seem dubious to people in general by distorting its true meaning; they have so little understanding themselves that they are unable to make any profound analysis of the subject. In the life of this world, such people make very tall claims, and because they lose nothing by denying the Truth, they mislead themselves and others. But on the Day of Judgement, when they are called to account and threatened with Hellfire, the Truth will overwhelm them. Suddenly they will start accepting those very principles which they had denied in this life.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.