But what they concealed before has [now] appeared to them. And even (6:28)

﴿بَلْ بَدَا لَهُم مَّا كَانُوا يُخْفُونَ مِن قَبْلُ ۖ وَلَوْ رُدُّوا لَعَادُوا لِمَا نُهُوا عَنْهُ وَإِنَّهُمْ لَكَاذِبُونَ﴾
[ الأنعام: 28]

6:28 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.

Their saying that they would believe if sent back is a lie.
Instead the statement that they used to keep hidden, which was ‘By Allah, we were not idolaters,’ has appeared to them when their limbs gave testimony against them.
If they were supposedly sent back to the world, they would return to the disbelief and associating partners with Allah that they had been prohibited from.
So they are lying in their promise that they will believe if they are sent back.

Nay, it has become manifest to them what they had been concealing before. But if they were returned (to the world), they would certainly revert to that which they were forbidden. And indeed they are liars.

phonetic Transliteration

Bal bada lahum ma kanoo yukhfoona min qablu walaw ruddoo laAAadoo lima nuhoo AAanhu wainnahum lakathiboona

Abdullah Yusuf Ali - Translation

Yea, in their own (eyes) will become manifest what before they concealed. But if they were returned, they would certainly relapse to the things they were forbidden, for they are indeed liars.

Safi-ur-Rahman al-Mubarakpuri

Nay, what they had been concealing before has become manifest to them. But if they were returned (to the world), they would certainly revert to that which they were forbidden. And indeed they are liars.

Page 131 English transliteration