Saru is able to contact Starfleet using a piece of technology scavenged from the Ba'ul, the predator species of his Kelpian race. Other installments of Star Trek: Discovery (2017) establish that Kaminar, the home world of both the Kelpians and Ba'ul, is considered pre-warp and thus off limits to contact with the Federation. If the Ba'ul are pre-warp, they should not have a subspace beacon capable of transmitting to a Starfleet vessel.