As someone who has been to Zermatt, let me assure you that not only are there no airplanes allowed on the ground anywhere near it, but cars are forbidden in Zermatt. this helps keep it a cute expensive resort town which is totally mountainous and has no extended flat spaces for things like runways. The closest really major airport is Zurich over 3 hours ( I could look it up, but it is not germane to my position) away. From Zurich, here are best flight options to Rio:
fly to Paris, connect to Rio (2 flight combinations)
fly to Sao Paulo, connect to Rio
fly to Frankfurt, connect to Rio
fly to Lisbon, connect to Rio
So, Madrid is not among the top options as a connecting point, although that does not mean that some AR18 teams did not use it. Maybe there were not enough seats on one of the better options. If you really have a desire to see these details, go to post#1 in the AR18 Primary Transportation Options thread.