The Efficient Market Hypothesis is actually a family of hypotheses. Folk usually pick the strong EMH because it's easiest to transform into a strawman and beat about the head and neck.
Having particular bearing on the Horowitz post is this remark:
Commentary often seems to presume that EMH,
or notions of market rationality generally,
provide some implicit or explicit guarantee
that current prices will be sustained, which
is false. No guarantee against asset price
volatility follows from either.
One of the bloggers I host gave a very good explanation of what the EMH is and what it actually implies: http://skepticlawyer.com.au/2013/05/29/bubble-trouble-all-in...
Having particular bearing on the Horowitz post is this remark: