Angling - high intensity?
I read the report. It's clear that the respondents have no idea of what constitutes 'high intensity' activity. In many active sports like running, rowing, cycling etc. high intensity is defined by a heart rate at 85% of optimum max which is derived using a formula (max = (220 - age?)). Such a heart rate would also have a measurable oxygen demand so that the lungs would also be stressed. At this level of activity you know you are doing it! You sweat and are breathing heavily, unless very fit. Indeed, this is how you achieve cardiac fitness through training and why these other sports need much higher levels of fitness.
Whilst active angling such as fly casting might be more active than some other forms of angling it still only constitutes low level activity because there is little increase in heart and lung activity, unless you are very unfit. It is tiring but sustainable over several hours, especially with a little practice.
The only times I've seen anglers under 'high activity' stress is through carrying too much gear (over 100lbs) over rough terrain (especially pebble beaches). Then they are sweating and their heart rate is raised. I know of a couple of cases of fatal heart attacks in these circumstances.