I was there on Sunday and to me this is the closest I have seen to illegal feinting. If you look at the run up he practically stops then continues another step to kick the ball. As a consequence Fabianski has moved off his line in anticipation. A lot of other penalty takers slow down but not stop, it just didn't feel right watching it.
According to IFAB, it's an illegal feint if the run up has been complete. So if he had feinted one step further in to his run up then it would meet the criteria.
There's no doubt that Fabianski was off his line and players from both teams were in the penalty area because of the stutter.