Brilliant video - though it's nearly 6 minutes long so I'll summarize:
Paul begins with the observation that any given heat of elite pool freestylers will have sometimes wildly divergent styles - not all of which are the classic "long, smooth" form taught in books, articles, and videos (which Paul diplomatically doesn't name). He points to Laure Manaudou and Janet Evans as particularly striking examples.
Further, open water swimmers and elite triathletes "seem to be a lot more choppy, punchy, and with a much higher stroke rate" than their pool counterparts.
The "choppy, punchy, high SR" style - despite seemingly breaking all the rules of [unnamed stroke technique philosophy] - is by definition efficient for these swimmers, because they are competing (and often winning) at the highest level of the sport.
Therefore, he says, "Efficiency in the water cannot be measured by the number of strokes you take per length, by itself. That would be a gross oversimplification of the freestyle stroke."
Personally, I use a higher SR in open water than I do in distance pool races, and I think it works for me.
As a counterpoint - and, I think, much less compelling argument - see this: