I think you will find that statement to have been true about 3-4 seasons ago, but it doesn't apply to the current gear.
Since the 160 (and now 161, 162). The tail-widths of the FW boards have increased dramatically; the same for nearly all board brands, not just Starboards. As a result, with even more power from the tail of the board we can run bigger and more powerful fins than ever before.
Although cutdown fins have been around a long time, they probably weren't as mainstream back in the 147/158 days, but now look at the R19 (as a 76cm cutdown), which is the "standard" foil to come with a 161/162 board.
If you want more power in a fin, generally speaking, you make it bigger (ie, a cutdown) or you make it softer. So now we are seeing more people using soft fins in all conditions, not just lightwinds. And some (myself included), using one fin for nearly all conditions - the lightwind fin that works great in highwinds.
So with the extra control we now have with our wider tail boards and everyone using softer and softer fins, the brands have worked on the nose shapes, making them thinner and more boat-like (specifically on the 162), which has allowed us to control these soft powerful fins downwind, where before it was uncomfortable with such power under your feet.
Another aspect that I suspect that plays a role is the "twist" in these fins. Specifically, in this topic we are talking about an R13-S for highwinds (vs an R13-H). I haven't tried this with a H fin, but with my M fins, there is less twist than in the S fins. Similar tortional stiffness, but less twist the stiffer they are. I suspect this extra twist in the softer fins allows them to depower as they flex when you sail downwind and help with control, whereas a M fin would be trickier downwind and a H fin even trickier again (that's a very basic explanation of how it works, but enough to make a point).
So, my point was.... Yes, an R13 70 S would be best for highwind. I personally use +8cm, but Remi likes +6cm rake. We are both 82kg. And now, because of the board's tail-widths, softer fins are better for highwinds. Maybe not S--, but a S compared to a H at least...
GetWindsurfFit.com // The Ultimate Windsurfing Fitness Program.