Read his post again - his issues were more around the disconnected inflator hose. That coupled with the 4 lb overweight will make one sink quickly as you have no way to compensate - means you have to fin up as he stated! What is there to be surprised about here?So, your optimal weight was some 16-ish pounds (8-ish kg), but you had serious issues with some 4 pounds (2kg) more? Given your stated experience, I'm a bit surprised that 2kg more than your optimal weighting would be a problem.
For me, 2kg is more or less is what I experiment with when adding another middle layer to my undergarments.