... an everybody w/ Canon 5D mkII can verify spot metering himself/herself based on the technique described
That's right. But even if verified as correct (because at this point they haven't been, as far as I know,) the results only matter to ETTR practitioners. And even then, no metering methodology was described.
I don't have a 5DMII, but unless its meter calibration is vastly different than other Canons like the 7D, I can tell you that if you spot meter a neutral, evenly lit surface, then a white-balanced RAW conversion of the resulting capture, with no additional processing, will have sRGB values of 100, 100, 100 at the metered area. That's 12.7% gray. The reason that's important is because it tells you exactly how much headroom you have above the metering point...which is just under 3 stops.
The article stated that the metering point gave 3.6-3.8 stops (or thereabouts) of headroom. But what about tungsten light (as you've mentioned) or worse, sodium lights? That headroom is gone in those instances. I don't practice ETTR and even I have to cut back my exposure by 1/3rd stop in those conditions.
The camera makers cannot base their metering systems on ETTR practitioners. The metering is (currently) designed to match film-based metering. And when metering as such, the processing has to allow for no clipping of reflected white under a very large range of lighting conditions. And if that requirement resulted in some unused headroom in daylight conditions, then so be it. ETTR practitioners can make use of the headroom when it's available, or just overexpose to their liking...whatever. But don't invalidate other's metering methods just because they don't fit with YOUR view of how digital should be metered.
I have an RMI Digital Gray Card that I use for both white balance and for setting exposure when in constant-light conditions. The reference itself is marketed as a WB tool only, as it's about 30% gray. But I determined an appropriate EC value by shooting some 92% bright copy paper under a 5000K light until RawDigger indicated saturation, then just placing the reference in the spot-meter area to see the difference in metering. That's my EC, which turned out to be about 1.3 EV (1.0 EV when the light gets red.)
Anytime I step into new light I will pull the gray card from my back pocket, perform a custom white balance, and if the light is constant, I'll just press my AE-L button to lock exposure. That's it...I'm done with exposure. I can photograph anything illuminated by the same light and know that I won't clip any reflected highlights. Could I have pushed exposure by 1/3 or 2/3 stops in some cases? Sure...but I rarely shoot scenes that are so static that I have time to figure that out, and I usually don't care about having 2/3 stop more noise. The only time it matter to me is when photographing my black dog. In that case I'll just use the ETTR that's built-into my camera...the expanded ISO range. I'll just set my Nikon to L1.0...instant ETTR.