كُلُّ ذٰلِكَ لَمْ يَكُنْ
Neither of those things happened.
In the narration concerning Dhū al-Yadayn, the question concerned whether the prayer had been shortened or whether forgetfulness had occurred. The response denies both named possibilities in that context; it is not merely a denial of their joint occurrence while leaving one affirmed.
The preposed kull makes the answer comprehensive with respect to the two alternatives raised. Reading it as only ‘they did not both happen together’ would not adequately capture that particular exchange.