The fact that the results are virtually identical is compelling enough for me.
Actually to me that test they did is proof that solder tim does make a difference.
When removing a soldered IHS, and placing waterblock directly on die with user tim, cpu temps go up by at least 6-8C, or up 10-15C if doing actual direct die with water, threads on xtreme previously linked with results of testing. However when removing the non-soldered, temps did not go up, clearly showing that non-soldered die attaches are less effective than soldered ones.
Problem is your beliefs that any time you remove a layer, you should get better temps. Try removing your heatsink and just blow air on IHS, since after all the heatsink and user tim is just another layer, or does surface area matter. If surface area doesnt matter, no point in reading any further.
The most important interface is tim1, since the die has very small surface area especially since relative hot spots (non-uniform temps).
Take a soldered IHS like E8400. Thermal conductance through die 125-150 W/mK >> solder tim 87 w/mk >> IHS 400 w/mk, now to a much larger more uniform area of heat before trying to cool relatively small areas with tim 4-6 w/mk.
Now remove solder IHS, and place user tim directly on die, temps go UP. Because now you are trying to cool small relative hot spots on die at very low thermal conductance of 4-6 w/mk with very small surface area. Or do so directly with water, again temps go up, on xtreme fallwind showed temps up 15C at stock TDP alone, any significant overclock and got throttling. Cant cool low surface area with water 0.58 w/mk it is futile.
Best temps in order would be
1) direct die >> solder 87 w/mk >> waterblock (now heatspreader)
2) die >> solder 87w/mk IHS 400 w/mk (to spread heat very quickly) before user tim 4-6 w/mk
3)and depending on non-solder die attach used in terms of conductance, bondline thickness, contact resistance, you might do better with removing IHS or leaving IHS alone. In that experiment you linked, nearly same temps, NOT SURPRISING at all.
If you want to prove soldered IHS doesnt matter (you cant it would defy physics), but if you want to show the difference, you need to use solder, not replace intel nonsolder die attach with even worse user tim1. I wouldnt have been surprised to see worse temps.
When one realizes that an extra layer of copper that increases surface area isnt always a bad thing, you can come to a different conclusion. Until then you can equally argue to cut the cooling fins off the heatsink, as they are just an extra layer of copper in way of air getting to copper base, or better yet get rid of that extra copper layer and user tim altogether and just put a fan on IHS.