I'm with Sword on this one. Sahz's appearance was a surprise that has not yet been explained, however in a gap of 100 years with Sahz already known to be traveling through time, there is nothing ridiculous about the idea of him turning up within those 100 years and flying as a military pilot. Especially since all stories should realistically be converging at that point (remember that each person using gates to travel through time seems to have a purpose behind their travels, the purpose seemingly being driven by Lightning). The upcoming DLC will probably reveal more, but my guess is that Lightning intended on him being there and therefore he was there.

I also agree with Sword that it doesn't matter anyway because if Sahz wasn't there some other pilot would have been and everyone would still have ended up in the same place (Sahz excepted, obviously) if Sahz was not there at all.

This is a fantasy game with characters. A Final Fantasy game, no less. Every single Final Fantasy game has key characters turning up at key times as a matter of coincidence. It's called good storytelling. Nobody wants the person who rescues the good guys to be an unnamed pilot of no consequence. Stop taking things too seriously! It's a story in a video game.