This - in the Chef's tips - is the important difference from most crumble recipes:
"Traditionally, the crumble is added on to the top of the cooked filling and then baked. I have found that the steam generated by the fruit compote creates a stodgy, heavy topping with only the very top being crusty. It will also be indigestible as the flour is not cooked. The technique that I propose is to pre-cook the crumble and finish baking it off to a crusty, crumbly, dry finish."