Really, the level of 2 and 4 mmol is a CONVENTIONAL level, but in many cases there are big differences. For example, when we push a strong athlete of 10000m to a MAX LASS of 8/9 mmol for 15-17 min, if he is well trained can run HM about 6 mmol. The difference depends on the characteristics of athletes : runners with more Fast Fibres with Oxydative attitudes can reach a higher level of lactate, as their engine works like a "turbo", using again a good percentage of the producted lactate as energy. Fibres of these athletes are able to use the best attitudes of Fatty Acids, developing the "Aerobic Lipidic Power". So, they are able to run till 3 mmol about, using essentially Fatty Acids preserving glycogen.
Paul Tergat (but also Paula Radcliffe, Jaouad Gharib, and many of the best runners) are of this type. They cannot run many marathons in one season, as their engine is able to develop more power, but the capacity of searching fuel eating their muscles (so, they are "self-cannibals"), if allows top results, is also damageous for thir body and structure.
Instead, there are other athletes rich of Slow Fibres, than can use in most part Fatty Acids, running at lower level of lactate, that can run many marathons in one year, without any danger, because never are able to run using fuel from their own muscles.
Regarding Da Costa, his 2 halfs were 64:42 + 61:23. If we suppose that he could have a value in HM of 60:20/60:30 (pace of 2:55 p/km), running the first Half at 3:04 pace he used the 94/95% of his AnT, of sure 2 mmol. But in this case, he preserved completely the tank of glycogen, and in his 2nd Half was able running about 98/99% of AnT, having yet his tank full of glycogen. Of course, at the end of the marathon his lactate could be 3.5 or 4 mmol, but it happened because of his distribution. If Ronaldo had run the first half in 61:23, he had used the 90% of his glycogen soon, not being able to use the best Fatty Acids when already without glycogen in his fibres, and could not run better than 2:09/2:10 the full race.
One of the reasons of the improvement of top perfprmances in marathon today, is the new system of running, using negative split. This system allows to preserve your glycogen, giving the opportunity of running the second half (if well prepared) very close your PB in HM. Practically, first Half of fast warm-up, second Half of competition.
About the idea of drug for Africans, is a so stupid fancy that doesn't deserve any reply. It's possible to explain very well every improvement knowing physiology, methodology, training and having the taste of investigating human possibilities, instead to close himself without trying to understand what is possible to do. Mother of morons always is pregnant.