The caveat here is that 4 mmol is just an average across the entire population. The average is that 4 mmol is the threshold, and runners can hold that pace for around 1 hour.
In reality, sprinters are more fast-twitch, and produce a lot higher lactate values. 4 mmol might still be a jog for a sprinter, but he can reach 20+ mmol in a hard effort.
Top endurance runners (let's say 1500-Marathon) have a very high % of slow-twitch muscle fibers. They produce less lactate across the border, and their max lactate is substantially lower.
3 mmol for Ingebrigtsen might as well be the pace that he can sustain just 30 mins, or an hour, and not a pace that he could sustain for 3-4 hours like the average runner could. So for him, that's still roughly his threshold, and he is doing the repeats at the correct intensity for him.
If a 400/800 guy would do repeats at 3 mmol lactate, that's certainly too small, since he achieves 3 mmol at a pace that is still just easy/moderate for him, whereas for Jakob and other slow-twitchers it's already a strong effort.
Pace-wise, the Ingebrigtsen's seem to do a lot of their repeats around 10k-HM pace, which is in line with "CV training" and also the paces that runners have been training for decades, without labeling them "threshold" or "CV" etc..