'Just as you forgot,' meaning: just as you abandoned. It was also said: the meaning of 'We will forget you' is: We will abandon you, 'the meeting of this Day of yours,' for you did not believe.
﴿كَمَا نَسِيتُمْ﴾ كَمَا تركْتُم، وَقيل: الْمَعْنى فِي (ننساكم): نترككم ﴿لِقَاء يومكم هَذَا﴾ فَلم تؤمنوا
