I should point out that the "fusion sky-mine" here is merely a small fusion reactor (thus that lovely FU logo) that powers an anti-grav generator, and can release its containment and detonate when an aircraft gets too close. It creates a nuclear-scale explosion without any nuclear-scale radioactive fallout, unlike what we would currently call a "fusion bomb".

There is an issue of neutron radiation, but that's short-range and (somewhat) easy to sheild against, particularly for a shape-shifting fantasy robot that can generate the appropriate armor at will.