The article's description of lacate threshold ("[the point where] your body is no longer able to convert the lactate as rapidly as it is being produced") would

The article's description of lacate threshold ("[the point where] your body is no longer able to convert the lactate as rapidly as it is being produced") would be the one I'd use but it doesn't square with the test, which appears to be measuring the point where he begins to rely on the anerobic system. It makes sense that they would want to test for that, since Karnazes isn't particularly fast, but he can run quite a long time, so testing where his anerobic system begins would be more interesting. If they'd wanted to find the point where Karnazes' body could no longer clear the lactate it was producing, they'd be able to find it. They mention testing his aerobic capacity, but I think they are referring to his maximal aerobic capacity (VO2-max).