Those students are full of crap or the extremely rare (like unicorn rare) individual with an eidetic memory. You can skim it or use it as a reference if you're using another source like Anki or quizlet, but actually reading it alone in one month for retention is not realistic for 99.99% of individuals. It can be possible to do this with smaller texts or references like Little Robbins, but not with comprehensive textbooks like Robbins or Harrison's.
For reference, my only textbooks for M2 year was big Robbins, a micro textbook, and a pharm textbook. 75% of our material came from Robbins, and it was an accomplishment if you made more than 3 passes on the covered material in 2-3 weeks, which was usually ~200 pages.