Your quote is about the fact that the market is too complicated to model accurately, and too fleeting to easily test a theory with any respectable degree of scientific rigor.  I entirely agree - it's very hard to show causation when any given correlation could be the side effect of a near-infinite supply of other factors.

Nonetheless, it is not a logical fallacy to suggest that there are causative relationships in the market.  They're there!  They're just hard to prove.  The best we can do is theorize, then see how well our theories predict the future.  That's the whole basis of analysis.

Difficulty clearly correlates strongly with price.  It's not direct due to all the other factors I mentioned, but there is a causative relationship somewhere in there.