That you need 15% power growth (which ends up being more due to primary and secondary stats increasing, by the way) per difficulty is just an assertion though, just like the existence of four different difficulties each being that much more rewarding than the previous one being necessary.
In their explanation, in particular, they were talking about how players wouldn't be satisfied with just a 1% stat increase but there's a huge room between 1% and 15% with even just going down from 15% to 10% making a large difference for overall power scaling since we're talking about an exponential curve here.
Assuming their 15% for 15 ilevel holds true, you'd have a 0.936% power increase per ilevel (1.0093615 = 1.15). Assuming the same ilevel increase as legion from pre first raid to last mythic raid (~120 ilevels), you're looking at 1.00936120 ~= 3.059 (+205.9%) power at the end. If, instead, it was 10 instead of 15 ilevels per difficulty, you'd be looking at 80 ilevels in total and thus 1.0093680 ~= 2.107 (+110.7%), or almost half the total power increase.
Now remember that this is purely the ilevel scaling as well and doesn't take into account that Blizzard also tends to introduce stronger trinkets, set bonuses and now azerite traits (previously artifact traits) which further increase your power over the course of an expansion.
Of course you'd still eventually want item squishes but this exemplary change alone would immediately cut their frequency by half.
Overall I'm just not sold on their assertions here, especially since those numbers weren't used during early WoW and I've personally enjoyed loot progression back then more than I do now despite having been largely affected by the, relatively speaking, small power gain in raids.