Amr ibn al-ˤĀs was married to Umm Kulthum bint Uqba but he divorced her when she embraced Islam. She then re-married Umar ibn al-Khattab.
The above was in the year 628 CE.
The narrations of his birth range from 583-589. Even taking the latest date of 589, his marriage at 11 would be impossible if we are speaking of the same Sahabi.
Could you give reference to that fatwa on IslamQA? I usually get my Fatwas from AskImam or DarulIftaa when completely necessary, but I prefer asking my local scholars. I highly doubt IslamQA would be wrong about this though. Perhaps you misread?