You really don't understand anything, being blind to see what changed in training.
1- With continue frequentation of the intensity and the gradual adaptation athletes have, when they have the same INTERNAL LOAD, during a period of 2-3 years they become able to run at 15"/20" per km faster in long run, and in 3"/5" faster long intervals. They become also able to REDUCE the recovery times, because one effect of training is to increase the permeability of the cell membranes, in order to remove the same quantity of lactate quicker.
2- This means that athletes can produce MORE lactate than in the past, being able to remove it in shorter time, and a percentage of lactate is a new source of energy, due to the Krebs cycle.
3- This is true for the long distances, but also short distances are benefitted by a higher threshold.
4- For raising the threshold, we have to use two different kinds of training :
a) continuous fast run of duration between 10' and 20' (for example, a race of 5000m)
b) repetitions of duration between 1'30" and 2', at a speed about 5% faster than the speed of the current LT (Lactic Threshold), with short recovery. In this case, the athlete start to produce lactate at a level under the LT, but starting every time before having total recovery, the level of lactate is growing, with increased accumulation. We call this type of training LACTIC RESISTANCE, and is connected with the ability to raise the LT.
In other words, the athlete runs, for example, 10 times 600m in 1'30" with 2' recovery. After the first can reach a level of
3 mml, after the second 3.5, after the third 4, at the end 10. This means he starts the repetitions in the Aerobic Zone, and ends in high Lactic Zone. This type of training contributes to raise the Threshold, and with a higher Threshold the athlete can run faster, at the same level of INTERNAL LOAD.
5- Don't be silly when you ask "If his best 400 and 800 times were back in 2017 why have they not improved?"
Because he never went to run again the distances, but arguably in the shape of 3'31" he could easily run 800m in 1'47".
You do the mistake of the most part of posters, who go to see the PB of every athlete, without considering WHEN and HOW MANY TIMES they ran the distance. I already wrote it before, but I need to repeat again, because you seems not to be very fast in understanding : the PB of El Guerrouj is 1'47"18 (1995) when able running 3'31"16, but in 1999 he ran 3'26"00 : do you think his PB in 800m could be the same, if he had run the event only on time ?
Mo Farah ran in 1'48"69 in 2003, when his best in 1500m was 3'43"17 : when he ran 3'28"81, his 800m still were at the same level ?
Caleb Ndiku has a PB in 800m of 1'52"6 in 2007, when 15y old. When was able running 3'29"50 in 2013, do you think that one was his level ?
Daniel Komen the king ran in 1995 in 1'50"8 and 3'34"63, but two years later improved till 3'29"46 : was the value of his 800m the same ?
Daniel Kipchirchir Komen ran 1'47"04 in 2006, when was also able running 3'29"02. How much is the difference between the double of his PB in 800m (3'34'08) and his PB (3'29"02) ? It's less than 5", and Komen in training did a lot of speed.
About the last statement, can be more correct to speak about "high volume of runs around the speed of the threshold", and this is something very different from the past.
When we speak about "high mileage", we need to understand what it means. For Japanese Marathon runners, 1000 km per week is a normal nileage, and 1400 km is considered "high mileage". Toshihiko Seko ran 480 km in one week, and his average was about 350. With their old and classic system, Kipchoge, with his 200-220 km, or Sondre Moen, were athletes of "low mileage".
While Haile normally ran more than 200km per week, Kenenisa rarely overtake 150, with a terrible intensity.
So, 150-160 km per week is not a "high mileage" when we speak about top athletes in aerobic events, and the ecceptions are very few, normally regarding athletes coming from 800m. Without that kind of "base" is absolutely not possible to run around the threshold 3 times per week, but when there is continuity in this type of training is the training itself increasing the ability to recover, nothing to do with doping.
Last thing : you have to change the question, when you ask "why the athletes of the past, already using high volume, were not able to recover training of high intensity in short time", in "The athletes of the past,also very strong, and using high GENERAL volume, but low volume of SPECIFIC INTENSITY, could become able to recover in short time if could use training of high intensity with continuity ?",
In this question there is the answer, and the reason because short distances don't improve, but long distances continue to grow, is that the key is the increase of the VOLUME OF THE INTENSITY (so, not the general volume, neither the intensity very much faster than the speed of the race).