They say they tried to get E. Coli to operate the enzyme (E. Coli is used often to make all kinds of drugs, like insulin) but it didn't work and got yeast to work with it.
So instead, they put the same genetic material into yeast which already had the necessary metabolic equipment to make the entire compound "de novo" (i.e. from scratch - just add sugar and yeast nutrients and it pumps out psilocybin). The downside is that the yeast's metabolic pathways chew up about half the product, so there is still work to do on making it more efficiently, but at yields of 600mg/L it's not exactly a bad start. (For reference, depression researchers tested the compound at dose of 30 mg for a 155lb man and it yielded psychoactive effects.)
Why even a 50% loss of product would be debilitating is a mystery. It's not like the yeast collects a salary, or like sugar is expensive. The author doesn't seem to realize how little of the stuff is needed for a dose.