While Matt Kirk did, more or less, have a 5 lb BPW, he was also doing massive miles per day. If you are hiking not-quite-40 MPD, you need even less insulation and layers than someone hiking "only" 25 MPD. His window of hiking was much shorter than average (!) and environmental considerations were a lot less. The AT can get nasty with the weather, but if you pick the "perfect" two months, it is reasonably consistent as well.
So, can SUL be done? Sure.
But under very specific conditions and parameters.
For us mere mortals, a typical ~4-6 month hike with a 5lb BPW would be very difficult. A speed hiker cranking out the close-to-40MPD has a better change of this gear category.
A thru-hike on a shorter trail with very specific environmental factors that don't change would be another possibility, too.
As a side note, I find once you get below 10lbs, I personally don't notice the difference. :)

