His point isn't how many actual points of SpD you get out of it, it's that the higher your SpD is, the less percentage difference it makes to the damage you take. Diminishing returns.
I understand that there are diminishing returns, but Pokemon like Conkeldurr aren't so specially defensive even with the Assault Vest that it matters.
Let's compare two attacks that are actually equivalent. We'll use Draco Meteor again, then another custom move that emulates that power hitting Conkeldurr's physical defense.
252 SpA Choice Specs Latios Draco Meteor vs. 252 HP / 4 SpD Assault Vest Conkeldurr: 300-354 (72.4 - 85.5%) -- guaranteed 2HKO
252 SpA Choice Specs Latios Draco Meteor vs. 4 HP / 252 SpD Assault Vest Conkeldurr: 220-259 (62.5 - 73.5%) -- guaranteed 2HKO
If both attacks hit for max damage, there'd be a 12% damage difference. If they both hit for minumum, that difference would be 9.9%.
The next attack is Psyshock, but made Dragon type and using 130 BP so all the numbers in play are the same except for which defense stat Conkeldurr is using.
252 SpA Choice Specs Latios Psyshock vs. 4 HP / 0 Def Conkeldurr: 331-391 (94 - 111%) -- 62.5% chance to OHKO
252 SpA Choice Specs Latios Psyshock vs. 252 HP / 0 Def Conkeldurr: 331-391 (79.9 - 94.4%) -- guaranteed 2HKO
If they both hit for max damage, there'd be a 16.6% damage difference. If they both hit for minimum, that difference would be 14.1%.
The differences in damage are pretty similar on both sides, though it is concerning that the theoretical Draco Psyshock would likely OHKO 4/0 Conkelduur without prior damage.
I still don't think that HP investment is solidly superior to SpDef investment. Diminishing returns are more apparent with Pokemon like Blissey or Assault Vest Tyranitar, but I think Pokemon like Conkeldurr don't have so much to worry about. If the investment makes a difference in a common attack OHKO/2HKOing him then I'd seriously reconsider my viewpoint, but honestly I don't feel like doing more damage calcs to find out right now. =P