Thanks to Animator and angusj for their help.

Now I understand a "Naked" triple. The triples that I have used before are presumably "Clothed" triples where 3 candidates only appear in 3 cells but there might be other candidates in those cells.

I actually used a different on-line solver to find the solution and it came up with a quadruple of 1, 4, 8, 9 in the other 4 cells of grid 8.

Does it always happen that there is a Quadruple (or Quintuple) in the cells that don't comprise a Naked Triple (or pair)? Thinking about this logically, I believe that these two conditions are two sides of the same coin.