More info on the Zoast:
Zoats were also the first and only case of Tyranids using actual mechanical technology, rather than just they bio technology. The Zoats are no longer used by the Tyranids, atleast as not part of the more aggressive forces that are the current Tyranid Hive Fleets (Behemoth, Kraken, Leviathan, etc). Many theories exist for the absence of the Zoats, like the Hive Mind no longer finding them useful and also thinking of their free will as a problem, so the Hive Mind tried to wipe them out and Hive Fleet Colossus were the remains of the Zoat who managed to escape.
However in my opinnion it's more likely that the Zoats are simply used as scouts, who are sent before the Hive Fleet or the Vanguard Ships to scout out galaxies and also make contact with the races there. They are not used as a fighting force thought.
Anyways, how couldn't the Zoats contact the Imperium? They had free will and if I remember correct, they communicated telepathically.