Interesting. I have seen other people use the Ingebrigtsens as an example of "threshold" training over polarized training. Typically people suggest "threshold" is Zone 2 (of the 3 zone model), although from what I have been able to understand, Lactate Threshold is the cutoff between zone 2 and zone 3. Seiler would be more likely to recommend training slightly above Lactate Threshold (actually right around Tinman's CV pace ironically) over training just below LT.
The rest of it lines up nicely with the avoidance of much VO2 work, keeping low intensity EASY (Seiler has mentioned seeing elite athletes walk up hills, similar to what your friend described of the Ingebrigtsen brothers), using HR as a training tool.
I wonder (just guessing here), if the slightly lower intensity used by the Ingebrigtsens might simply be explained by the fact that they are running and a lot of the examples/studies he uses involve cycling, skiing, or rowing.