Why do serpentine traces (supposedly) entail decreased ESD immunity?
On page 18 of the referenced document, it mentions that long serpentine traces can contribute to radiated EMI and decreased ESD immunity. 

As for the radiated EMI portion, this may be due to the fact that "long serpentine traces" can unbalance the circuit and potentially cause an emissions problem when used with unshielded RJ-45 cables. This is really a layout consideration. 

As for your primary question on ESD immunity, I personally don't think think the person that wrote the excerpt on page 18 knows what they are talking about. More than likely what the author meant was "RF immunity" instead of "ESD immunity", which is a big difference. 

This makes sense since that if there is a design guideline that could cause emissions problems that due to reciprocity there also could be immunity issues as well.