My opinion, cardio is short for cardiovascular. So while I would consider hiking exercise and definitely burning calories I don't count it as cardio. For me cardio=elevated heart rate, hard to breathe, good amount of sweating.
When I go hiking it's mostly just walking and site seeing. I might occasionally have to hike up something challenging but overall it's a relaxing thing for me.
I know a million ppl will disagree with me and that's fine, that's why I said my opinion in the beginning because this is my personal opinion. I just needed to draw a line somewhere.
Like if walking on a treadmill is cardio, then is grocery shopping cardio? Or right now I'm walking down my stairs to my truck, is this cardio? They both involve the same effort as walking on the treadmill lol