They needed a controlled type of exercise for the study, they chose weightlifting, randomly or for whatever reason. To ensure that all of the subjects were at the same baseline for that type of exercise they chose people with no weightlifting history (to eliminate variables).
This does not mean or infer that if you weight train regularly you will not experience elevated enzymes post exercise.
"G&H How are liver dysfunction and vigorous exercise both linked to raised transaminase levels?
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3104191/
MS Healthy individuals have normal levels of trans-aminase enzymes. Although the levels that constitute “normal” are currently under debate, most people who are healthy have levels that are recognizably within the normal range.
However, just as these markers do not discriminate among liver diseases, they do not discriminate among several different sources of inflammation. Thus, raised levels can be traced to muscular inflammation as well as hepatic inflammation. People who exercise heavily (eg, weight lifters, marathon runners, soldiers who perform heavy-duty labor) can have abnormal transaminase levels, as can those suffering muscular trauma, such as myocar-dial infarction or even surgery in which some muscles are cut. The normal repair process in these cases engenders inflammation and raises transaminase levels."