[tpallarino]

If people trade based upon information asymmetry, then the fact that Bitcoin Cash actually had miners, didn't crash fatally, and was capable of sending transactions constitutes an enormous change in the information landscape.

Pre fork BTC price + risk premium ~ BTC post fork + BCH

Pretend like the fork actually taking place and working successfully as comparable to a company's earning reports. The second that news comes in, it prices itself in almost immediately.

[davidmanescu]

Very true - neither the person who wants BTC (but has no opinions on BCH) nor the BCH buyer wants to take the risk that the other coin will drop before he/she can sell it. In other words the risk premium isn't zero.

However a rational (impartial?) actor should be willing to take that risk if the premium is big enough. That could mean buying pre-fork or selling pre-fork if there is an obvious opportunity. Which means in an efficient market the risk premium should be zero "on average". It's hard to reason about averages with one sample but that sample was distinctly not zero!