I do believe that it greatly weakens the line, but by 50% is a bit of a stretch. I'd like to see some break testing to back that up.
This knot looks like a slip knot. Getting very tight with a load and very hard to untie. It will also not work for a halyard unless the sail wasn't at full hoist, and it looks bad. Doesn't offer much. It would hang up on shrouds as much as a bowline with being harder to untie. And most jib sheets are much larger than they have to be, so breaking one is rare.
Merit 25 # 764 "Audrey"