Was Quo Vadis written expressly to be an accurate historical account? I know Sienkiewicz did a lot of research before writing the book, but it was published in 1895... Any book written on the basis of historical knowledge anno 1895 will by necessity be very inaccurate today.
In any case, even at the age of twelve I didn't read it as a historical work, nor do I imagine anyone would today. I took it as a good story, very well told. Hence the criticism, if correct, is really of no consequence.