Umar’s ignorance about Quran

Hafiz Abdur Razzaq Abad bin Hamid and Ibne Mudhir have narrated through their chains of narrators from Dauli that:

A woman was brought to Umar, who had delivered a child after six months of pregnancy and Umar ordered her to be stoned to death. Her sister came to Ali Ibne Abi Talib and said:

 “Umar wants to stone my sister to death. I adjure you by God that if there is any excuse for her, let me know.”

Ali  said: “There is an excuse for her.” So the woman recited: “God is the greatest (Takbeer),” which Umar and those around him heard. Then she came to Umar and said: “Ali believes that I have an excuse for my sister.” Umar sent someone to Ali  and asked what the excuse was. Ali  replied: Allah Almighty says:

"And the mothers should suckle their children for two whole years…”     (Surah Baqarah 2:233)                                        

And also says:
 “And the bearing of him and the weaning of him was thirty months.” (Surah Ahqaf 46:15)

And God says:                          
“And his weaning takes two years.”  (Surah Luqman 31:14)
 
Now, even if we keep aside this report, it can be concluded that the period of pregnancy in the verse:  َ

"And the bearing of him and the weaning of him was thirty months.”  (Surah Ahqaf 46:15)
 
…is supposed to be six months, [because, the period of weaning is 24 months and subtracting this from thirty months, six months remain.

At this point Umar released that woman.

The narrator says: After that we came to know that the woman delivered another child after six months’ pregnancy

(Al-Musannaf, [7/350, H. 13444])