I think the point is that you need to be aware of what you are effectively doing when you use a skinny string on a bow. Say you start with a 50lb bow using a 500 grain arrow with a 14 strand dacron string. You are at a nice 10 gpp arrow that is considered safe by virtually everyone. My 14 strand endless loop dacron strings, with silencers, weigh about 160 grns. You change to a skinny sting, and low and behold, you need to drop 50 grains of point weight to tune the bow and get good arrow flight. Great you are now getting really fast arrow flight and great trajectory.
You are still at 9 gpp for the bow so you are still safe right? Maybe not. My 8 strand, padded, D97 endless loop strings with silencers weigh about 90 grns. So this effectively removes about 130 grains of weight from the arrow/string combination. If you removed that 130 grains from your arrow and stayed with the original string you would be at less than 8 ggp. Not considered safe by most. So I don't think the 9 gpp with the skinny string is safe either.
Yes, I lot of people have broken bows using modern strings, but I don't think the sole reason is the string material. It is because they have not given consideration to the entire system that goes into the mechanics of the bow arrow shot with the effective change in arrow weight being one of the primary culprits.
For the record, In my 40 plus years of shooting recurve bows I have broken one expensive custom bow, designed for fast flight due to an accidental dry fire. Incidentally, it had always been shot with a 14 strand dacron string. So I believe when bad things happen a bow can break with any type of string.
I now shoot all my bows with skinny modern strings with the rule that my arrow/string combination will weigh over 12 gpp of bow draw weight. When following that rule, the best shooting bow I own (or ever owned) is a 45lb, 1966 Kodiak bought off the auction site for $140 (comparing very well with high dollar custom bows I own). I am shooting it with a 560 grain axis shaft with 250 grain head, and 8 strand endless loop d97 padded to 16 strands in the loops.
I am with Steve in that, with the availability and cost of older bows, I think it is worth the risk to shoot skinny strings on the old bows for the improved shooting characteristics alone.