Hernia surgery is not only routine, but quite straight forward. Mesh implants are easy to handle and offer a good long-term outcome. It's one of the first interventions a beginner in the field would learn to master. Also, I am not sure about Sierra Leone, but in Europe and the US it is commonly done as a keyhole surgery, so it requires some dexterity with the tools. The study chose to examine open surgery, which is easier to perform but leads to more post-operative complications. Also, they only looked at elective surgery, so only at cases when the hernia didn't cause further problems demanding a more timely treatment, such as bowel constriction or even inflammation or incarceration. These can be much more challenging to treat properly.
All that being said, the authors purposefully sought to compare performance for a type of surgery that doesn't offer many obstacles for somebody new to the job. I guess it helps to identify tasks that highly-skilled MDs are freed up from doing when they are already scarce in a given location. But it's hardly an indicator that much what they do could be done by others, as some would probably like to believe.
All that being said, the authors purposefully sought to compare performance for a type of surgery that doesn't offer many obstacles for somebody new to the job. I guess it helps to identify tasks that highly-skilled MDs are freed up from doing when they are already scarce in a given location. But it's hardly an indicator that much what they do could be done by others, as some would probably like to believe.