The issue was Binaries. These are unhedged for IG and dont work like other instruments. Most of 'adz2k11''s profits came from binaries. I believe the outright Index trades were done to disguise the fact that this was a 'hit' on IG binaries.
Binaries would be a losing proposition for IG, but they apply their own position sizing on punters. Winning punters are restricted by bet size. Losers can bet as big as they like. This allows IG to make millions in profits from them.
If every net winning binary punter was allowed to bet at £400 a point like this 'adz2k11' chap was doing, then IG wouldn't make any money from binaries.
In a good month i can make 10K betting £10 pound a point on binaries.
If IG were to let me bet £400 a point, that would be £400K, but they dont let me bet anything like that, its £10pp if i want to get in and out with zero delay on entry and exit orders (and zero delay/zero rejections are vital in order to cut losses).