Why would you say thay he was illiterate, the Quran (e.g. 25:4-5) and the hadiths say otherwise and he was a successful merchant which implies some literacy and a lot of interaction with Christians and Jews. Denigrating the prophet seems a strange stance to take.
Why would you expect any one single person to have made such a claim? Multiple people over many years would have narrated various passages and stories.
does merchant in deep past need to be literacy ? in that time Arabs may exchange trades , with goods , much than currency .(gold or silver) , of course there was interaction with Christians and Jews , plus his neighbor is a Jew and one his wives was Christian (Mariya)
for that verse of Quran 25:4-5 , God recit/sort the claims of the disbelievers (you),
and deny it in next of verse 25-6 .
And those who disbelieve say, "This [Qur'an] is not except a falsehood he invented, and another people assisted him in it." But they have committed an injustice and a lie.And they say, "Legends of the former peoples which he has written down, and they are dictated to him morning and afternoon.
"Say, [O Muhammad], "It has been revealed by He who knows [every] secret within the heavens and the earth. Indeed, He is ever Forgiving and Merciful."
The idea that there was no exposure to the beliefs and teachings of people in the area is ludicrous.
EDITED TYPO Errors
I agree but should that base on facts ,
random picking without using references as (know language "Arabic" ,historic ,and explaination "tafsir") , is like driving car without gaz , so it's stop you in your limit .