I agree, most people do not recommend transplanting because Sunflowers are particularly root sensitive, like morning glories. They tend to stop growing as well or die if their roots are damaged or disturbed. If you want to transplant use bio cells with individual cells for the seeds. then once they are about 3 " with their second sets of leaves you can gently remove from the cells and plant wherever you'd like.
But planting in a pile of compost lets the roots really spread out with no restraints so there's really no way to ensure you don't damage the roots and end up killing your flower.
Happy Gardening :)