From left arm around with Starc's action, he's not swinging it 'away' towards first slip in a traditional sense, he's swinging it back-to-straight because of the angle. It's a really awkward thing to wicketkeep to, on the basis that if Starc loses one down even slightly down leg it's angling a long way down and you're throwing yourself at it and hoping. With the amount of short stuff on offer, standing straighter is perfectly reasonable to ensure that a leg side strangle is in catching range.Yeah but you know Starc can swing it away from the right hander from that angle. So you are still going to the offside unless the ball deviates off the wicket. Mean, the first movement is still the regular one, at least it was for me with our left arm opening bowler back in school days.
It means that an edge to first slip is going to be a much thicker one with much more deviation from the original trajectory than if the ball had got there from a right arm over release, and the wicketkeeper is going to be far more unsighted. I think it's a tricky catch where needing a dive, in it itself, isn't a failure of technique. Paine executed very badly because he didn't look ready for it and he's not an exceptional 'footwork' keeper.