If you get into the position Bettinelli is in and try and shift your weight from a neutral position to you left foot. The first thing you'll notice is the body shift towards that side, your knee and shoulders will dip slightly (not a lot but slightly) towards the side you've shifted your weight to.
You see that movement from Bettinelli after the ball is past Spence, not before the ball is hit or the moment it is hit. It is my opinion that Bettinelli gets caught flat footed and reacts too late.
Is he the sole factor in the goal being conceded? No.
Is it a difficult save to make? Yes.
Should he do better? Yes.