<< Oddly, 1% of respondents had "no opinion" as to whether they'd flown before. >>

1% seems to be pretty low. Some people have flown before but not in a long, long time. Rather than trying to remember if they did take a short flight in 1973 or 1982, they'll say "I don't know for sure", and it's recorded as a "no opinion". Surveys are never perfect, even if the question is black-and-white like this one.