In this video at 15.30 he explains that a possible explanation for sahaba reciting certain variations of the Qur'an could be that those variants were then abrogated after the final review of the Qur'an (during Ramadan, Jibreel and the Prophet pbuh) https://youtu.be/GCU_2WX_sBA
This is just one explanation and another is that they were only explaining the verse.
But if it was an abrogation, then How would we know if we are reciting abrogated qirat or qirat that are still valid, as I thought Uthman wrote it in the dialect of Quraish, so why do we recite with different dialects? What if those dialects are abrogated?
Or do the minor differences between Uthmani manuscripts mean he still acknowledged that different dialects were allowed?