Is there a meaningful distinction to be made between the system and the content?

[Satinavian]

For me, the big distinction between 5e D&D (for instance) and PF on this spectrum is the presence of a "hard" default setting in the core books. 3e had a bit of this as well--it's "default" made pretty strong assumptions about the settings. Not quite as tightly detailed as Golarion, however. PF, for better or worse, fair or not, is identified (maybe just in my mind) with Golarion, while D&D incorporates multiple "conforming" settings, each relatively different. Even Forgotten Realms differs from the presentation of the defaults outlined in the PHB/DMG/MM in many ways. For instance, default paladins don't need a god at all. FR paladins get their powers directly from specific gods and have alignment requirements. FR's cosmology is also slightly different (the World Tree, rather than the modified Great Wheel). Etc.

Yes, but you have paladins and they do get spells and you have clerics and they do get channel and so on. While D&D is not married to a single setting, all D&D settings are pretty similar.

If you put D&D at 20, where is GURPS ? Or even GURPS with only the fantasy-appropriate books ?

Also while Pathfinder only has one setting, it is not mechanically closer to it than D&D (what surprise, considering it is the same system at its core). It is not as if D&D doesn't have exactly as many setting specific rules even outside of setting books (Red Wizards, Suel Arcanamachs). I have to do exactly tthe same work to port a setting to Pathfinder as i have to do to port it to 3.5. I don't think it is particularly useful to put them at different points on the scale when you can effortlessly use Pathfinder for every D&D setting and D&D for Galorion.

Generally my experience is that D&D does Fantasy settings that are not specifically made for it or inspired by it pretty poorly. I have found it way easer to apply other fantasy systems, even those that were not made to be universal. I mean, decades ago i have seen a group that found it easier to convert to Shadowrun of all things for some fantasy setting after having tried and failed to run it with AD&D2. (Nowaday with the internet, they would probably find something more fitting more easily)