The enrollment rates are absolutely central to the discussion. Demand is drastically higher in the US - partly due to a young population, partly due to the strong social stigma of not having a university education. Higher demand creates higher costs.

The figures given are for all postsecondary education. I'm not sure why this is a controversial statistic. The younger US population would mean that even if the same percentage of graduating secondary school students go on to university, the US enrollment percentages would still by higher. Coupled with a higher percentage of students that seek university degrees after finishing secondary education (~70% for the US as compared to ~50% for Germany as per some quick Google searches) and a total enrollment figure of twice the rate of Germany is about what one would expect.