Until airline deregulation brought prices down, most air travellers were travelling on business; it would be unsurprising that you saw a lot of business suits. I often wear a business suit on airplanes even now. However, the late 60s was also the hippie era, and among the non-business travellers you saw a lot of hippies, who certainly did not dress to fly.  The article you cite is naturally going to display photos that support its thesis; the author is scarcely going to show you plane loads of grungy hippies.

Young boys don't pay much attention to fashion; however, their mothers do. If the norms of the time had called for getting dressed up to get on an airplane, you can be assured that my mother would have made sure I was dressed up before I got on an airplane.