The words Hard & easy are ver subjective as you know.
When the new wave of sudokus came about in 2005, Newspaper puzzles rarely needed anything beyond doubles.
The "Superior" puzzles published by the Sunday Times in the UK were puzzles that needed triples or an x-wing to solve. These were considered the most difficult in publication for a while.
The term "Superior techniques" came from there. These can be compared to the current APPRENTICE category used here on these fora.
Techniques like quads or swordfish were considered beyond superior
, techniques have evolved & increased in number, hardenss remains very subjective. The term "Superior" IMO remains unchanged.
tarek