This is correct. 6:5 is unbeatable. "Naturals" are a major reason for the advantage, but this is because you bet more when you know you are more likely to get a natural. Playing a "full index" system allows you to make (or save) a lot of money on all the other plays, especially doubles and insurance. Perfect insurnace play is a huge advantage, but it is very difficult to play insurance perfectly (you also need to side-count tens). In a "full index" system, insurance still accounts for around 50% of the total advantage of an expert player, even more if you side-count aces and adjust for this.