Honestly, I don't have an answer as to why. I have read some articles, one was the one with the person who came up with this recipe and he initially added the enzyme in the fermenter but said next brews he would add during the mash.
Not real sure if there is a benefit of adding in the mash or fermenter.
In the fermenter it will take longer to work due to lower temps.
There are a couple guys on this forum who brewed a brut and added their enzyme as I outlined above and in three days were down to 1.000 or below.
Most people that I see brewing bruts are adding the enzyme in the mash and during cool down after the boil so that's what I opted to do.