Presumably, they were historians.
Here's the quote from the novel....

" "Historians, you think," said Miss Tilney, "are not happy in their flights of fancy. They display imagination without raising interest. I am fond of history--and am very well contented to take the false with the true. In the principal facts they have sources of intelligence in former histories and records, which may be as much depended on, I conclude, as anything that does not actually pass under one's own observation; and as for the little embellishments you speak of, they are embellishments, and I like them as such. If a speech be well drawn up, I read it with pleasure, by whomsoever it may be made--and probably with much greater, if the production of Mr. Hume or Mr. Robertson, than if the genuine words of Caractacus, Agricola, or Alfred the Great."

"You are fond of history! And so are Mr. Allen and my father; and I have two brothers who do not dislike it. So many instances within my small circle of friends is remarkable! At this rate, I shall not pity the writers of history any longer. If people like to read their books, it is all very well, but to be at so much trouble in filling great volumes, which, as I used to think, nobody would willingly ever look into, to be labouring only for the torment of little boys and girls, always struck me as a hard fate; and though I know it is all very right and necessary, I have often wondered at the person's courage that could sit down on purpose to do it." "

Okay, does anyone know anything about Mr Hume and Mr Robertson? What did they write?