I'm surprised this has been so provocative for people. I don't think that it was Will or Ryan's intention to re-assert SUL as an important social category, but rather, as the article says, as a mindset. If you fixate on the concept as a public form of distinction, then it quickly becomes a petty way to differentiate yourself by the gear you carry.
When I come back from a trip, I just say I went backpacking. Its not important for me to differentiate in most situations. However, when I'm packing and planning for a trip, the mindset is everything. Similarly, if I'm planning a big trip with a friend, and I have a 6 lb base weight and they have a 10lb base weight, you can bet that I'll be doing what I can to balance that out or take more of the load depending on their fitness. Its not because I care if my partner is 'UL' or 'SUL', its because depending on the terrain and the mileage, it might be the difference between finishing the trip or bailing, or at least the difference between pleasure and suffering. I'm skiing a peak in RMNP tomorrow with a forecast of T-storms in the early afternoon. Its a good day to leave behind the thermos.
So, yeah, the attitude between UL and SUL is of the same category, but there is a difference in degree thats makes a practical and experiential difference in the mountains. Knowing the types of trips that Will does, and having traveled the same terrain, I've got no doubt that the difference between a 6 and 9 lb base weight is an important one, not for blabbing about here in the forums, but to carry on your back along with 4 days of food while you scramble up a remote pass in the Weminuche. It might literally be the difference between fun and suffering.
Its likely that I'll never use the word M-SUL again in my life (unless I hike with one of ya'll!), but for sure its a poignant concept that I already use. For me its just about being as light and mobile as possible while maintaing a healthy degree of safety and protection in the mountains. With modern lightweight equipment, I can carry a kindle, a fry pan, an SLR and a spacious tent and still be under a UL 10lb base weight (which is awesome!), but for the majority of the trips I've been looking at for this year, I will definitely leave that stuff behind in favor of a faster and simpler setup. For me, 6ish lbs is a good target weight.