McDonald's has worked very hard to standardize the look and taste of their hamburgers. Of course, there is some variance. Not every hamburger is exactly the same. The cook may have squirted a bit more sauce on one or sprinkled a bit less lettuce on the other. But due to their standardization process every hamburger is almost the same as any other in any McDonald's around the world. That's the idea.
IELTS, like McDonald's, standardizes its examiners. They all have a set of questions they are to ask. They all have a set procedure they are to follow. They all have a rubric they are to measure by. They all receive training and monitoring and double checking to make sure that they are all as close to standard as humanly possible.
Is it still subjective? Yes. But the degree of variance due to subjectivity has been greatly reduced through their system. I will not contest that.
However, my contention is that this level of control is not sufficient to consistently measure slight changes in English speaking ability from a period like one semester of training. The issue is extremely important, meaning the pass or fail of thousands of students across China.