Kapie

S2000 is correct:
This is a typical case of illicit minor syllogistic fallacy . The minor term  is distributed as the conclusion but not in the process.
Argument : the January effect is true . This proves that a weak form efficient market does not exist  .ergo a strong form efficient market cannot exist too.
An explanation of the illicit minor argument is :
All  Dogs have tails                                                                              
A cheshire cat has a tail too.
So the cheshire cat must be a dog.
The fallacy:
All strong form efficient  markets are weak-form efficient as well.
So in order to prove that a strong form efficient market cannot exist , it is sufficient to prove that a weak form efficient market does not exist.
Forgive me.

soverby

The January effect is an argument against weak-form efficiency (that prices quickly reflect market data), but certainly is not an argument against strong-form efficiency (that markets quickly reflect all data, public and private).  The January effect has nothing to do with insider information.