Excerpt from their methodology page:
focus exclusively on institutions’ overall academic research and reputations and not on their separate undergraduate or graduate programs
So this isn’t just a list of undergraduate institutions.
Anyway, I find a “global” ranking not very helpful given the vast differences between education systems across the globe. Not just in terms of academics, but also differences in college culture, focus, cost, etc.
I imagine the number of students and families comparison shopping across the globe is a very small, very wealthy group.