The most recent leaked images have mentioned that the Knight Warden is an Adeptus Mechanicus Imperial Knight. This suggests that it is a Lord of War for the upcoming Cult Mechanicus Codex as well as for the Imperial Knights codex. So will these two codices being sharing the Knight? or will there be rules in the Imperial Knight codex allowing it to be shared with other allied forces.
One source here is saying that he has seen no entry for the Knight Warden with the Cult Mechanicus. Of course time will tell.
via an anonymous source on Faeit 212
No, there is no low entry in the cult mechanicus dex, nor a formation or detachment that uses one. I know that contradicts what the poster says, so maybe there has been some last minute update. there should be something on it in the imperial knight dex. there is a battle congregation that makes up the cad for cult mechanicus, which includes up to 8 troops, 4 elites, 2 heavy 1 hq, and a fortification