Arcturus was observing the Red Queen, who controlled the entire Obscurus as much as possible to act stealthily and approach the Mind Fleet Leviathan and their Flagship, where the Elder Brain was most probably located.
The Psion-Disruption Device was already prepared as Arcturus ordered the Red Queen to control everything several times, because they needed to be absolutely sure that everything was working as it should be.
If the Psion-Disruption Device couldn't work as it was supposed to, the results could be catastrophic for the Imperial Defense Forces, especially. Without the usage of the Psion-Disruption Device, they couldn't take out the Psionic Abilities of the Mind Flayers and greatly weaken their fleet.
Even right now, as they were heading towards the Flagship of the Mind Fleet Leviathan, Arcurus could see tens of starships of the Eternal Imperium of Asurya being destroyed in the exchanges between them and the Mind Fleet Leviathan.