Is it true that the UCs, even though they claim to be holistic, generally place greater emphasis on grades and test scores?
Yes it is true that UCs place greater emphasis on grades and test scores, but that doesn’t necessarily make them any less holistic than any other schools. Most schools do actually, regardless of their holistic claims.
If you look at the common datasets for the UC’s, GPA/Test Scores and Course rigor are considered very important in their admissions. Also notice, essays are considered very important to important also. See section C7 on the common datasets below:
http://www.aim.ucla.edu/profiles/cds.aspx#cdsA
https://www.sariweb.ucdavis.edu/commondataset/cds20142015.pdf