That is only true if you are trying to hunt them (or have them hunt you) outside of the event, at least according to the DE post on the topic, they will spawn in any regular mission etc etc:
Defeat the Grustrag Three
The Grustrag Three are disrupting the construction process. Lure them out and eliminate them!
During The Pyrus Project, the Grustrag Three have a chance of spawning in any regular mission (with enemies that are higher than level 3) on Earth, Mercury, Ceres, and Saturn whether or not you are marked by them.
I have now done multiple missions at all of these locations as well as invasions to see if I could get them to spawn. I had enough materials for the other required stages long ago. I haven't seen them once. Maybe I am just unlucky, perhaps there is another factor bugging them out for me, I don't know but neither their normal spawn triggers (the invasions) nor the stated "any regular mission" trigger seems to be working for me.
DE - obviously there is some confusion, numerous persons are saying they have spawned for them during invasions but not during normal missions. That is not the stated requirement in the announcement. Can you clarify / check this?