OP, the big thing people miss when they hear Jakob is running 26 miles of threshold per week is that he & Norwegian runners in general define threshold differently than Daniels. Where Daniels defines threshold as 1 hour pace (on average 4mmol/l of lactate) Ingibregtsen defines threshold as somewhere between traditional T-pace & M-pace (3mmol/l of lactate or less). When you cross the lactic threshold, the consequences are dramatic, requiring exponentially longer recovery & allowing you to sustain much smaller volumes of the pace. By backing off the pace just a bit, he can run 2-3x as much "threshold" in a week.
Also, I have never seen the Ingebrigtsen's run a continuous threshold run, always interval style as this is much less demanding on the body. Even when they run 20-25x400m=5k pace for their afternoon session on double threshold days, they're keeping lactate levels relatively low despite the faster pace because of the frequent, short breaks & can recover from the session quickly.
That said, notice Jakob has lost the 1500m world championship twice in a row. My suspicion is that this high volume, threshold model is great for producing even-pace runners who can time trial to world records in non-competitive fields. However, I wonder if Jakob has been outkicked for gold by Josh Kerr & Jake Wightman because the Norwegian threshold model deprives runners of the finishing speed that more traditional approaches provide middle-distance runners.
I'm interested to see if Jakob can continue to win at the Olympics/World Championships as people understand that he lacks the finishing speed of many other world-class middle distance runners. Personal bests & world records do not win championships.