“I’d strongly dispute that statement! Not for the cities on OP’s list,but as a general statement. There are large swathes of the US that are certainly much less ethnically mixed than at least urban areas in the UK.”
Apologies, I should have been less definitive. Here are the relevant statistics for UK vs US students: The average at UK universities is a 75% white student body (see https://www.bbc.com/news/education-44226434, these stats appear to be for 2016 or 2017). For comparison the average at US universities was 57% white in 2014 (see https://nces.ed.gov/pubs2017/2017051.pdf Fig 19.2) and was falling by about 1% per year.
Also note that the diversity of the student population (which is what I was referring to) is typically much greater than for the population as a whole. For example the population of Iowa is 91% white (https://www.census.gov/quickfacts/ia), but the University of Iowa is only 71% white (https://provost.uiowa.edu/sites/provost.uiowa.edu/files/wysiwyg_uploads/cds_1718.pdf) and Grinnell College is 52% white (http://web.grinnell.edu/institutionalresearch/webdocs/GC_CDS_1617.pdf).
So yes there are some universities in the US which are less ethnically mixed than universities in London (where 45% of students are white). But hardly anywhere in the US is as white as Oxbridge (80% white), let alone Scottish universities (90% white).