I haven't read it either so don't know what they said or how the testing was done. I will say we've done a LOT of testing in this area and any differences seen are 99% mass related and the other 1% is drag due to string diameter. How much string mass effects arrow velocity depends on where that mass is located. 10 grains in the serving area is the same as adding 10 grains to the arrow, 10 grains in the loops is like adding about 3 grains to the arrow.
If a person used under built Dacron and over built modern strings, yes the results could be minimal. If compared strength to strength, IE: the same pounds breaking strength between strings, the results would be MUCH greater then 5 fps.
Energy is divided up between limb mass, string mass, and arrow mass. Increase the mass of any one and less energy goes to the other two and vice versa.
5 fps at the same gr/lb??? That's only the same as short drawing or over drawing more then 1". Anyone that's been shooting more then a week can "see" how significant that is when we do it in the field. It'll be interesting to see who the author is, bet there's an underlying agenda some where. There was an article written many years ago called "Anatomy of a string", they came to similar 5-6fps conclusion but "spun" it as being insignificant looking at fps as a %. Looking at fps is like looking at RPM in an engine, it doesn't relate to horsepower in itself. This one was written by a bowyer who didn't like folks using non-stretch strings on his bows for failure reasons....O.L.