I read that capillary action doesn't work between two dissimilar layers of soil. The water "perches" on the first without draining into the lower layer.
If that top layer gets waterlogged due to poor drainage, nothing can germinate and roots will drown.
Perhaps a thin top layer (like 1/4" or less) quickly becomes 100% waterlogged and then drains out by simply overflowing. If it is thin enough, or coarse enough, air would still penetrate enough that seeds and roots get the oxygen they need.
I always use a very coarse mix now that guarantee4s aeration & drainage no matter how much I ov er-water. Of c ourse that is TOO coarse for fine seds that need light and need to stay on the surface. In that case, I sprinkle a very thin layer of vermiculite on top of the coarse mix so that petunia and lobellia seeds don't fall into deep dark crevices.
My hope is that the vermiculite is thin enough and "wicking enoguh" that it does establish a capillary coinnection to the coarse mix under it (whic h does have SOME fine fibers of bark and peat).
