I’m just telling you what my experiences have been. I myself at one time didn’t think her recommended grain size would make that big a difference, but it does.
You asked for advice, I gave it. Simple truth is I myself personally wouldn’t use that size sand in anything other than just putting a 1cm max layer on bare bottom glass.
I’m not that educated, anything I’ve learned has come from hard knock school of total fail and being corrected by my friend who is biologist/freshwater ecologist/aquaculturist with multiple degrees. Walstad’s book totally snowed me also when I 1st read it. Then he explained that she’s instructing how to set up a littoral facies in a aquarium. In particular the submerged upper edge in this diagram, as you go deeper in this picture the more anoxic substrate becomes.
Also explained why pore size and circulation in substrate is so important in setting up proper oxic and anoxic layers. There is a fine line between a beneficial mildly anaerobic layer and a stinking/putrid anoxic layer that rots plants from root, engulfs your tank with algae/Cyanobacteria usually only found in profundal zones of aquatic ecosystems and even at times releases a built up methane/sulfide bubble layer into tank that can if severe enough wipe out every fish, snail, shrimp in your tank. I’d prefer to know that my tank isn’t ever going to go that place if all I simply have to do is make sure my sand is proper size.
Dennis Wong’s section also touches on pore spacing on his website. To me this section and following one on layers and microbial action should be required reading for anyone setting up a aquarium, not just planted aquarium.
Most people who set up a soil tank would also be months ahead in reaching a viable/stabile system if they would pre-leach soil of excess humic/organic compounds and mineralize soil for a couple weeks before placing in aquarium.