So what would be wrong with buying 1 bitcoin in the market (564 at that time) and selling a futures contract to sell at 865 (at that time). You would lock in a tremendous rate. Also no "risk". What would stop you?
Nothing would stop you from locking in this rate, and this would be a straightforward strategy for someone calculating their position's value specifically in fiat terms. For someone calculating their position's value in BTC terms, however, they might be concerned to avoid erosion in their BTC holdings should BTC rise significantly against fiat -- thus the strategy of coupling the futures position with an additional long position in BTC.
It all depends on aims and risk tolerance.
and why do you need a fund size 4 times as large as the amount of bitcoins deployed?
You don't. However, I was drawing the original article out of a bag of tools I had used in fund management, where it would rarely be appropriate to sink all of a fund's capital into a single strategy.