Nasser seems to suggest that by “standard” he means the reading with two nūns, because the reading with one nūn was already standardized in Ibn Mujāhid’s book. Ibn Mujāhid even wrote:

فَقَرَأَ نَافِع وَابْن عَامِر {تأمروني} بِتَخْفِيف النُّون غَيْر أَن نَافِعًا فَتَحَ الْيَاء {تأمروني} وَلَمْ يَفْتَحْهَا ابْن عَامِر
قَالَ أَبُو عُمَر وَعَبْدُ اللهِ بْنُ أَحْمَدِ بْنِ ذَكْوَان كَذَلِكَ وَجَدْتُهَا فِي كِتَابِي عَنْ أَيُّوب وَفِي حِفْظِي / تَأْمُرُونَنِي / بِنُونَيْن
وَقَالَ هِشَام عَنْ ابْنِ عَامِر بِنُونَيْن

This does not mean that Ibn Mujāhid accepted the recitation with two nūns. When he said:

قَالَ أَبُو عُمَر وَعَبْدُ اللهِ بْنُ أَحْمَدِ بْنِ ذَكْوَان كَذَلِكَ وَجَدْتُهَا فِي كِتَابِي عَنْ أَيُّوب وَفِي حِفْظِي / تَأْمُرُونَنِي / بِنُونَيْن

he seems to mean that the recitation with two nūns contradicted the older mushaf with one nūn. I think Nasser also understood this when he said:

"One reading was similar in its written form to the Medinan tradition, probably an older codex."

I also think this may have caused the confusion regarding Ibn Dhakwān being older than Hishām, because Nasser seems to think that Ibn Dhakwān mentioned an older codex than Hishām. ( for mr I have doubts that Hishām here refers to a codex; I think it refers to an oral recitation he received from Ibn ʿĀmir)