Not surprisingly, I disagree. RBR had undisputedly a fast car during Vettel's years, that is not disputed. What is however disputed is mantra that it was the fastest car. Horner would suggest it was Vettel's driving style which made him fast in the turns, good enough to win him a lot of races.
Not too long ago I read litany by one Hamilton's fan that Vettel was lucky in 2 of his WDC, and the other 2 he had the fastest car. So, there you go, another "expert" opinion, and another half-truth perpetuated ad infinitum on various forums. This fellow of course either forgot, or haven't noticed in his zeal all about disparity between Mercedes and the rest on grid for past 4 or 5 years.
More importantly ...IMO... prior hybrid era, other teams could redesign elements of the car to match RBR, and challenge Vettel. Aero pieces (or chassis) do change all the time, thus teams could realistically compete.
Since 2014 this has changed. Mercedes has best hybrid design, and with all restrictiveness in place, and freezing design first year of introduction, Mercedes drivers had virtually guaranteed victories for years to come. There was no way with this idio*** point system anyone could fix their power unit.
When Hamilton lost to Rosberg in 2016, Tony Kanaan said about Hamilton (paraphrasing) "he drives in championship of two cars, comes second best, and bragging about it". That about sums it up for me as well.