Just as Kung Fu is a skill that requires a great deal of training and talent to master, any activity ending in Fu is an activity performed in a masterful way. You can say "How is your carpentry-fu?" or "You really showed those snowboarders your pipe-fu!" Nixyfu is an internet-chat shortened version of "Unix-fu", which is used to describe the activity of pwning a unix problem with your madskillz.
By pwnnig that haproxy, John earned the thanks of his peers for his nixyfu.