Yes in theory the timing could still slip with the bolt capture fitted. But as yet no one has reported a slip with a bolt capture fitted.. (fingers crossed)
From a mechanical stand point (and having discussed this with some very experienced mechanical engineers), looking at likely friction levels, it's not really a bolt capture it's a hub capture if you get my drift.
The centre bolt itself is massively tight with huge friction levels on its threads inside the crank end, so the capture really secures the hub to the bolt rather than the other way round.
The main bolt needs way over 600nm to remove it and as very little mass, so it's going nowhere easily. I never bought the idea of the centre bolt backing out personally. Even if some strange torsional vibrations cause the hub and bolt to vibrate at different frequencies and a slip occur then this capture will help to prevent that.
The hub assembly by comparison is a heavy old lump of rotating damper / cam drive / oil drive / belt drive which will be trying to move separately to the crank the whole time, relying only on small friction discs to hold it in place.
By locking the hub to the bolt you are securing the weaker point (low surface area hub & timing gear friction rings) to a stronger point (main bolt with large thread friction area).
That's my theory anyway
