I have her book 'Grow your own garden' and this is what she writes:

We do not use compost to cover seed at all, but a fine, even sprinkling of dry grit. This replicates the conditions the seed would experience naturally, allowing light & warmth to get to work on it, & provides emerging seedlings with sharp drainage, while retaining moisture under the surface where it is most needed by developing roots. In all aspects of gardening the best advice is to emulate nature: this is especially true for growing from seed.

Hope this helps answer your question.

Grit/sharp sand in compost greatly improves drainage so stops cuttings rotting off before theyve formed roots.

I usually use a mixture of 3 parts potting compost to one part sharp sand as a medium for cuttings on my plant nursery.