Shape- hmm- the current E-109 does have a lot of sidecut for a XC ski....
But- I think it has a lot to do with snow conditions and turning technique...
Gamme the elder is literally forcing those Gamme 54s to turn in those videos- forcing them through a combination of striding (in his instructional video he is clearly lifting the inside ski right off the snow), and lunging/jumping into a carved turn. The ski has to be quick, light, and responsive to do this "easily"- not that I would describe that lunging/jumping technique as "easy"- you got to be in peak condition to do that all day.

Regardless- Gamme is not using the "modern" technique of simply putting a ski, with parabolic sidecut, on edge- weight it, to engage the edge- and then watch 'em turn! Mike- you've described it well- a ski like the S-98 "wants" to turn. Traditional XC skis don't "want" to turn- one has to use technique to make them turn.
Most every ski in Gamme's chart is a traditional BC-XC ski (except for the Storetind)- because of this, I suggest that his rating of a ski's turnability is related to how effectively they can be made to turn.
Would be interesting for him to test and plot the S-Bounds and the Madshus XCDs on his chart (all of the S-bounds would end up with the Storetind; the XCDS maybe a little less "turny").
Interesting that the Norwegians have little interest in the S-Bounds/XCDs. They do sell them in Europe...just no interest in Norway?