The reason this has happened is his previous training was Jack Daniels based (right? or am I misunderstanding?) which means a majority of his race prep training was done at Hard Tempo (roughly half marathon to 10k effort (slower pace)), 5k pace, and mile pace. These all train type IIX fibers maximally (half marathon - 10k also trains IIb fibers very well, the other paces only slightly do this).
This Norwegian Training System focuses on workouts from marathon (+) to 10k pace/effort, which trains the type IIb fibers very well, but not only that, it also transitions your type IIx Fibers into Type IIb, which in turn increases ability at any race between the Marathon and the 3k directly (raises your Critical Power/Anaerobic Threshold and vVO2 Max). This is also why you may feel less "explosive" during true base/threshold training. Your top end speed should remain relatively similar, however, your anaerobic tolerance decreases for anything over roughly 200m (150-250m depending on your training and genetics). If you don't maintain some form of speedwork, your top end speed will also begin to deteriorate.
This brings me to why the Norwegians include hill sprints. Efforts faster than vVO2 Max that leave you "huffin and puffin" (ie 20x200m HARD Hills w/ 2x time jog rec.) actually begin to transition Type I fibers into Type IIx fibers. In general, as a long distance runner, this is bad. This means your fatigue resistance will decrease and you will become more glycogen dependent the more you do this.
Here's the catch : The more Type IIx fibers you have, the more potential Type IIb fibers you have.
This is important to understand, because if you do things right, you will increase your potential as a runner. Many people point to SirPoc doing "traditional" training and then improving massively with threshold training and while this "threshold fitness" is more suited to long distance racing and he could've done well without the harder intervals to begin with, these hard/grueling workouts were a different type of "base" for this threshold work.
This is the reverse of "I tried Lydiard high mileage, but I didn't race well, so I dropped my mileage and went back to my hard interval training and ran the best I ever have!". But in this case, SirPoc dropped the hard intervals (and didn't increase mileage a ton) and improved quite a lot with the threshold focus which primarily transitions the Type IIx fibers into Type IIb fibers.
Now, are these hard workout necessary to PB in 5k and up? the simple answer isNO, however, your potential will not increase, you will only squeeze out more from your potential instead of adding to it. If you were to add a tough anaerobic workout faster than vVO2 Max you may become weaker aerobically, but you will perform better at the 3k and under, while also increasing your potential at the 5k and up.
Now, you might ask, "What about type I fibers? Don't I need those?" and then answer is awesome : You recruit/gain more Type I fibers from running below your Aerobic Threshold (marathon pace ish). This does not take away from your Type IIx fibers! But don't forget to train your Type IIx fibers (sprints) otherwise they will deteriorate.
In short, exceptional Distance Racing between 3 hours and 3 minutes requires exceptional Type IIb Fibers, you get Type IIb fibers from Type IIx fibers, and you get Type IIx fibers from Type I fibers. This is why threshold training is so productive! Threshold training allows you to use your potential (in distance running), everything else increases your potential.
A side note, this is why anything between Aerobic Threshold and Critical Power/Anaerobic Threshold is not always a good idea for 800m runners. As a true 800m runner, you don't want extra Type IIb fibers, you want primarily Type IIx fibers, but you still want to train the Type IIb fibers you have (endurance is still important in an 800m) and you do this by training between your Critical Power/Anaerobic Threshold and your vVO2 Max, hence the 800m "Base" consisting of Easy Running below the Aerobic Threshold, vVo2 Max Work, and Sprint work. This then transitions into Lactate Tolerance work and such, etc.
Continuing off the 800m training idea, this is why you cannot run a (relatively) good 800m when you are in (relatively) great Marathon to 5k Shape. The most obvious case is Jakob Ingebgritsen who, when running at the Prefontaine Diamond League meet, ran 3:43 in the Mile and 7:23 in the 3k. This puts him at approximately 26:20s 10k shape (in the same condtions, time trial, and rested shape). I would then estimate he could "only" run 1:45 (ish) in the 800m, which is incredibly fast, but relatively slow when compared to knocking on the door of a world record in the mile.
The thing here is, Jakob Ingebrigtsen is only "slow" because of the way he train, if he wanted to be a good 800m and 1500m runner, he could be, but he would have to give up his world record ability racing shape at anything above the 3k, because he would have to convert back to Type IIx fibers (3 - 15 min race pace training).
A counter-example to Ingebrigtsen is Nick Willis, who was able to run very fast 1500m races (sub 3:30), a decent 800m (1:45 when his 1500m PR was 3:32), and a lethal kick in championship races. When Nick Willis tried to transition to the 5k, he failed. This was partly because he was older, but mostly because he didn't spend enough time transitioning his fibers to Type IIb. He most likely did similar "Miler" training with more volume, rather than a slower pace/effort. This is also why Gjert has been known to say, it may take years for his program to fully work.
Anyways, sorry for the ramble, but this has been on my mind a lot lately and I am open to discussion on any points I made as I'm sure I have made an error somewhere.