I think this is a great thing for Viking to do. We've had two experiences where people over estimated their abilities and it caused issues. My husband is very athletic and was on a Viking kayak excursion. The kayaks were two person kayaks and a woman was put in his kayak that had never kayaked and should not have been attempting to do the excursion. The waters were rough and he had to do all the work for two people and worry about her not causing any safety issues. It was very unpleasant for him. On another cruise line excursion where we both were snorkeling, an elderly frail woman had snorkeling on her bucket list, which was totally unrealistic. The guys running the excursion were very kind to her and basically carried her for a while, but then had to take her back to shore which messed things up for the rest of us.
I don't think our experiences were the norm. Most people recognize what their abilities are. Some "demanding" excursions might not be demanding, but there are those that are!