My question is, how does he qualify to state that whatever he saw was 'alien'?
I'll take this opportunity to comment on this sentence: "This wasn't just the world's first UFO sighting — this was the birth of a phenomenon, one that still exercises an extraordinary fascination."
Kenneth Arnold wasn't the first one to see UFO's. He wasn't even the first to call them 'flying saucers', the Japanese government beat him to that by 400 years. Arnold wasn't even the first post-WWII eye-witness. There was a previous wave of UFO sightings in Scandinavia in 1946. There was even a rock band named "The Foo Fighters". Foo Fighters is what US pilots called UFO's during WWII.
For the previous 50 years before WWII, there were some, but not many UFO sightings. The Fatima phenomenom has been investigated as a UFO event. It had many of the same properties. There was a huge wave of "Airship" sightings throughout the US in the 1890's, including landings, including contacts with the Airship occupants. All was reported in the papers of the day and seen by tens of thousands of people.