From what I read, Walstad used the potting mix. She has some algae issues with it, but this didn't bother her at all.
Other aspects of Walstad's sytem are to make sure the tank is shallow with lots of water surface and never change the water, only top it up.
I think OFL's system suits fish-keeping specifically, in that you change the water 50% 2x pw for the first couple of weeks, then 50% x 1 pw for a few more weeks. then finally, ease it off to once every few months and make sure there's a balance of light and nutrients.
I think if you sift out the really woody parts it'll be just fine.