Niessing, so far as I know, never used platinum for tension sets.
Well, they did and they do. And I think the important difference
actually is not the alloy but the fabrication. On Niessings website
they say that the rings are cold-forged, then the slit is cut to the
size of the stone and the diamond inserted.
In Frankfurter Allgemeine Zeitung, an article described how Walter
Wittek, the jeweller at Niessing who “invented” the tension ring in
1979, did it: Niessing die-cut the rings from 500 grams platinum
(Pt950 and Pt960) ingots. The strength comes from cold-working the
metal, imho, not from the alloy.
And, to add to my former comment about protection, the Niessing site
states: “And in 2001 it was also legally approved as artwork and
copyrighted.”