These arguments probably would not last as long if you actually track down the source. Because chances are they have addressed your objections. From the study, which is linked from the blog post which can be found above:

"It is important to note that no attempt is made in figure 1.3 to “adjust for quality differences” in tertiary educational experiences between countries. Of course, not every university around the globe is a Harvard, Stanford, Cambridge, or Indian Institute of Technology, so invariably considerable “skill aspects” are not included in fi gure 1.3. Such comparisons are significantly beyond the scope of this policy analysis. However, following the axiom that tertiary training is what principally enables individuals to quickly grasp new complex subjects and therefore makes it easier to train them on the (especially services-sector) job, it seems evident that in terms of describing the overall level of high-skilled workers in different countries’ workforces, any impact of quality differences among universities will be swamped by the quantitative differences in tertiary skill uptake depicted in figure 1.3.16"