Here the pronoun (it) refers to their denial mentioned in the previous verse. Whereas, this pronoun (it) refers to the Holy Quran in the verse:

 وَإِنَّهُ لَتَذْكِرَةٌ لِّلْمُتَّقِينَ

(And indeed, it is a reminder for the righteous.) (Al-Haqqa: 48)

Allah (swt) uses the same emphatic tools in the two verses (And indeed).

(… لَحَسْرَةٌ… ) (…regret …): indicates a pain in the heart that a person feels when he/she is suffering from a calamity that is unavoidable. Allah (swt) says:

 وَقَالَ الَّذِينَ اتَّبَعُوا لَوْ أَنَّ لَنَا كَرَّةً فَنَتَبَرَّأَ مِنْهُمْ كَمَا تَبَرَّءُوا مِنَّا ۗ كَذَٰلِكَ يُرِيهِمُ اللَّهُ أَعْمَالَهُمْ حَسَرَاتٍ عَلَيْهِمْ ۖ وَمَا هُم بِخَارِجِينَ مِنَ النَّارِ

(Those who followed will say, “If only we had another turn [at worldly life] so we could disassociate ourselves from them as they have disassociated themselves from us.” Thus will Allah show them their deeds as regrets upon them. And they are never to emerge from the Fire.) (Al-Baqara: 167)

Indeed, the loser is the one who lost his worldly life along with the hereafter as a consequence of his/her disbelief in Allah (swt), the Prophet (saws) and the Holy Quran.