We did not include USA in the international comparison because, as you correctly suggested, there is no comparable national cluster policy there. However, we use USA in Table 1 on basic conditions as the baseline reference for the three countries.
This should be backed up with evidence (data) and preferably rephrased, such as ‘recognised R13 research universities’, or ‘elite’ with reference to the German federal government’s programme of university excellence (Exzellenzinitiative).