وَقَدْ أَضَلُّوا كَثِيرًا
And indeed they have led many astray.
meaning, by the idols that they took for worship, they mislead a large number of people. For verily, the worship of those idols continued throughout many generations until our times today, among the Arabs, the non-Arabs and all the groups of the Children of Adam.
Al-Khalil (Prophet Ibrahim) said in his supplication,
وَاجْنُبْنِى وَبَنِىَّ أَن نَّعْبُدَ الاٌّصْنَا
مَ
رَبِّ
إِنَّهُنَّ أَضْلَلْنَ كَثِيرًا مِّنَ النَّاسِ
And keep me and my sons away from worshipping idols. "O my Lord! They have indeed led astray many among mankind...") (14;35,36)
Allah then says,
وَلَا تَزِدِ الظَّالِمِينَ إِلاَّ ضَلَلاً
Grant no increase to the wrongdoers save error.
This is a supplication from him (Nuh) against his people due to their rebellion, disbelief and obstinacy.
This is just as Musa supplicated against Fir`awn and his chiefs in his statement,
رَبَّنَا اطْمِسْ عَلَى أَمْوَلِهِمْ وَاشْدُدْ عَلَى قُلُوبِهِمْ فَلَ يُوْمِنُواْ حَتَّى يَرَوُاْ الْعَذَابَ الاٌّلِيمَ
Our Lord!
Destroy their wealth, and harden their hearts, so that they will not believe until they see the painful torment. (10;88)
Verily, Allah responded to the supplication of both of these Prophets concerning their people and He drowned their nations due to their rejection of what he (that Prophet) had come with