Sure. But what exactly could GW gain from all of this? Or Kings, for what it's worth.
As I said, it seems to be lawyers making themselves look important, nothing more. "Astra Militarium" would be easier to protect than IG, but the term "Imperial Guard" is already so established, it won't go away just like that. Leaving aside the irony in GW sueing others for IP infringement, it kind of feels like a step back that will affect the recognition value of a lot of armies' names.
"Imperial Guard" is a name that, in the tabletop market, has become so closely affiliated to GW that it's hard to think of something else. After about 30 years, however, how many people will know what an "Astra Militarium" is? We still have gamers who are unaware of what the Adeptus Astartes are!