What they are actually saying: Given one correct quoted sentence, the model has 42% chance of predicting the next sentence correctly.
So, assuming you start with the first sentence and tell it to keep going, it has a 0.42^n odds of staying on track, where n is the n-th sentence.
It seems to me, that if they didn't keep correcting it over and over again with real quotes, it wouldn't even get to the end of the first page without descending into wild fanfiction territory, with errors accumulating and growing as the length of the text progressed.
EDIT: As the article states, for an entire 50 token excerpt to be correct the probability of each output has to be fairly high. So perhaps it would be more accurate to view it as 0.985^n where n is the n-th token. Still the same result long term. Unless every token is correct, it will stray further and further from the correct source.