"They do not worship except as their fathers worshipped" means: except for what their fathers used to worship before; meaning: they used to worship idols. "And indeed, We will give them their full portion" of the punishment "without any reduction."
﴿مَا يَعْبُدُونَ إِلا كَمَا يَعْبُدُ آباؤهم﴾ أَيْ: إِلا مَا كَانَ يَعْبُدُ آبَاؤُهُمْ مِنْ قَبْلُ؛ أَيْ: كَانُوا يَعْبُدُونَ الأَوْثَانَ ﴿وَإِنَّا لَمُوَفُّوهُمْ نَصِيبَهُمْ﴾ مِنَ الْعَذَابِ ﴿غَيْرَ مَنْقُوصٍ﴾.
