Do a bit of search on engineering safety margins for cordage. IMO, Sgt is being liberal. A margin of 5 to 1 is for non-human support
, so 5 to 1 will be the ratio of strength to maximum working load for such as tow ropes and straps. 10-15 to 1 is the safety margin for human loads.
On the other hand, and it is a strong hand.
- The load is almost shared by two cords, each bearing about half the weight.
- Engineering conservatism appropriately assumes that the cord will be terminated in knots which will compromise the strength by 40-60%. On average ,then, there's a need for cord that is twice as strong as its maximum expected static load. However, and in contrast, you will find in HF everywhere, descriptions of proper splicing methods for (hollow braid) Amsteel, which result in no more than a 10% strength loss from terminations. So, that source of risk to safety is minimized. Further, IIANM, the rating on Amsteel from Sampson Rope is already of a properly spliced line.