Sick wikipedia shit bro. If you're going to go there, look up the actual article for Ambient music and educate yourself. Sigur Ros and Explosions in the Sky are GREAT fucking bands, but they're experimental rock bands, nothing more nothing less.
Sorry, I'm not going to get into a silly semantics argument with you after this post. The fact that SOMEONE considered them to be ambient-ish to the point of including it in their Wikipedia entry shows that, at worse, it's a fluid definition, or that "ambient" can be interpreted in different ways.
Feel free to search any number of hundreds of reviews on the band too. Here's one from NPR, for example - http://www.npr.org/templates/story/s...toryId=4834623
Explosions in the Sky artist tags: http://www.last.fm/music/Explosions+in+the+Sky