They specifically referred to "failure rate", so it does take into account the population of each model/make. However, that's assuming they know what "failure rate" means and have correctly calculated it.
They also don't describe sufficiently how they got the data - was it self-selecting? If so, it's basically worthless. If not, it might have some legitimacy.
Also, as one of the commentators pointed out, there are two variables: time (four years), and mileage. There is no mention of how they adjusted for the latter, or if they did.
The results might be l