Obviously, it's impossible to tell exactly what Arnold saw. It could have been alien spacecraft, it could have been geese, or he could have made it all up. But what we do know is that there's no evidence of strange crescent-shaped craft flying past Mount Rainier that day. Explaining eyewitness accounts scientifically is impossible without supporting evidence because we don't know what we're supposed to explain, if anything. When it comes to UFO stories, the important thing, to me, is determining whether something constitutes evidence of anything extraordinary or not.
To me, there are two different types of UFOs: the LIZ UFO and the high-information UFO. A LIZ UFO is something that remains unidentified because of a lack of data, while a high-information UFO is something deemed anomalous despite the existence of scientifically testable evidence, for which no mundane explanation exists. The problem is that we have a lot of LIZ UFOs, but not a single high-information UFO. In light of this, the "there's nothing to look at" hypothesis seems like the obvious null hypothesis.