I think original meaning was that freestyle has the most complex movements (although I personally think Breaststroke is more complex), while Butterfly is the most tiring stroke.
If you compare both strokes using these definitions I think it's hard to argue with the text. Even the most accomplished butterflyers in the world can't swim with an efficient stroke for 1.5km.
I also think Butterfly can be much more elegant than Freestyle. It's not easy to make Freestyle seem easy, but it's much harder to do the same with Butterfly. When I see someone swimming Butterfly in what seems to be complete relaxation, it is much more impressive (and therefore looks more elegant) than seeing them do the same with Freestyle.