It is interesting. The article makes the point that bees have immunity to the harmful nature of Metarhizium and that it cannot survive the heat of the hive. However the paper doesn't yet address the question of whether bees can tolerate the engineered version. Nevertheless there is potential for it as an anti-mite agent.
Phil Chandler, some years ago, was adding deep litter floors to his hives with organic matter in them... he said, at the time, that he believed that these floors were helping the varroa loads in the colonies on them ... who knows, he might well have inadvertently stumbled onto something ....