"Low" doesn't mean bad. Sure, practice is better than highlighting/rereading, but I expect that practice fails if the student makes a mistake that can be cured by re-reading. Also, practice takes a lot longer (costs a lot more), so it better have higher impact.

Highlighting is an indexing technique, and maybe a marginal improvement to reading. It's a piece of a solution.

I've practiced highlighting on and off over the years, but it's never been for memorization. I've mainly used it to separate out critical passages to speed up and simplify future reading.

Absolutely this. I use it on my Kindle all the time for this same purpose, and come back to some of them when I know the Chapter they are in and I need to refresh a concept or phrase.

One round of highlighting is almost worthless on its own. BUT. "Progressive summarization" (term coined by Tiago Forte) starts with broad highlighting, followed by 1-2 more rounds of increasingly refined highlighting, and finally rewriting material in own words based on the last round of highlights. Leads to deeper understanding and recall etc.

I once purchased a used textbook wherein over 75% of the text was highlighted.

Stopping at highlights alone might be the problem. Migrating highlights to rewriting it your own words for spaced and periodic review can be helpful.

When I was in law school, I highlighted in different colors to mark different parts of cases (facts, procedural posture, legal holding, etc.). I found this to be very helpful, both when I was marking up cases, and when I was reviewing the cases and able to quickly find the different elements. This is especially important in law school, where professors cold-call students and expect them to know various facts about each case at a moments' notice.