He admitted this in his New Year post:

This is a rare opportunity to be with the family, dress up in Santa Claus and just have fun.

Perhaps he is Santa for two different families — one is his own, and one is us :)

Because it messes up with the contest environment. Visual clues are kind of important. It is not a super big deal, but I still want to have some idea on how competent is guy that I try to hack (whether to expect some unreadable mess or maybe something more readable or whether I can expect this guy to make a silly mistake with bigger probability) or whether this guy that unexpectedly solved some hard problem is a total random or not

Time is nothing but a stubborn illusion.