The pen and paper hadith

So according to sahih Bukhari 114 Ibn Abbas said, "When the ailment of the Prophet (ﷺ) became worse, he said, 'Bring for me (writing) paper and I will write for you a statement after which you will not go astray.' ButUmar said, 'The Prophet is seriously ill, and we have got Allah's Book with us and that is sufficient for us.' But the companions of the Prophet (ﷺ) differed about this and there was a hue and cry. On that the Prophet (ﷺ) said to them, 'Go away (and leave me alone). It is not right that you should quarrel in front of me." Ibn `Abbas came out saying, "It was most unfortunate (a great disaster) that Allah's Messenger (ﷺ) was prevented from writing that statement for them because of their disagreement and noise. But the prophet was an Umi(he couldn't read or write) so why did he ask to write?