The "100 IU/100 grams" starting point is with fresh mushrooms, before they were dried. It's not clear to me from the article whether "46,000 IU/100 grams" is per 100g of the fresh mushroom input or per 100g of the dried mushroom output. Mushrooms are ~80-90% water so it makes a big difference.
It makes a big difference, but even if you removed 90% of the fresh mushroom's weight without changing vitamin D levels, you'd still only have 1000 IU / 100 grams.
