The whole Chicken's neck vulnerability claim is among some delusional Chinese members here, with no military thought.
To enter Shiliguri from Chinese border side, one must scale mountain range on an average of 6000m high.
In other words, the Chinese infantry will have to first get acclimatized to mountain peaks, then when descending downwards must again get acclimatized to conditions of the bottom of the peaks, this usually take a month for average human body.
After that lies the task of expelling/overrunning existing Indian military forces at Siliguri.
Even if the above becomes possible magically, India can send massive reinforcements from mainland, faster than China.
Plus any Chinese retreat from Siliguri is not possible, as they would have scale the mountains again.