I have no idea about author's background or about his other writings, but the conclusion quoted by you seems absolutely correct.

I do not understand why you say that it lacks supporting evidence.

Before this conclusion, the author has explained the system of national exams and of distribution into high schools and inside high schools, that ensures that the best students are grouped together and also that the best teachers are assigned to them.

There is no doubt that even if the average level of education is bad, this system guarantees that the best become very good and much better than students with similar native qualities who have stayed in high school in the middle of less capable colleagues, while being not taught things deemed too difficult for the general population. Moreover, the author has mentioned that the state provides rewards for good results at the International Olympiads, both for teachers and for students.

I do not see what more evidence could be brought. In my opinion the conclusion of the author is well supported and it explains why these students compete successfully against students from much bigger countries.