I've seen kids run, doesn't mean it's not physical demanding to run.
It's not, if you don't want it to be. Old grandmas run.
Are you saying surfers don't in general take surfing to physically demanding limits? If not, then I'm not sure what your point is. That you could dial it down if you wanted to even though surfers don't, but they could if they wanted to?
Physical demand on the body is relative to the person. Kids paddling into small waves is much more demanding for them than for an adult. Same for grandmas.
Yes, they do 20cm white water waves, which is very easy. Difficulty increases by 5 if you go to the 2 m green water waves, where it usually takes you 10-20 mins of constant paddling through 2 m waves until you're behind the break.