The argument that Zilkhstan hasn't been mentioned yet but still can be mentioned later on is not a valid one. That can be said of literally anything. Maybe Lancelot & Guren will mention that Kallen had a Kermit the Frog plushie. Should we add that to Kallen's page as well? Of course not. What isn't mentioned doesn't exist, and if it does get mentioned at a later point, the wiki page can be updated accordingly at the appropriate time.
Saying that words like "canon" don't belong in articles is just your personal taste, it is not an objective fact. Articles shouldn't be written around subjective tastes of individuals, no matter who they are. Deleting facts based on taste very much goes against the soul of a wiki. Furthermore, saying that the word "canon" shouldn't be used because it can be retconned is, just as above, not valid. Anything can be retconned. Lancelot & Guren can retcon Kallen's hair colour. Should we therefore not mention hair colour? Of course not. A wiki gets updated when retcons happen.
Since you prefer not using the words "canon" or alternative universe" theonly remaining option is to remove the paragraph entirely because as it stands not is is factually incorrect for the anime.
If later additions to the tv series' universe include Zilkhstan and how they reacted to the Zero Requiem we can update