ILB canonicity was in question for a long time, but was accepted a while back. By the story, it's presumed that the activation of the artifact at Chawla Base sent out a massive EM/slipspace disruption (or transmission) which signaled to Covenant where Earth was. There was also the Flea, a small Covenant spy AI that hijacked Melissa (the AI of the Apocalypso) which was programmed to locate Earth and somehow inform the Covenant.
So, AFAIK it's still sort of up in the air as to what precisely happened, but the story strongly implies that it was the artifact (de?)activation that led them to Earth. This is another reason I really really hope this plotline is continued! :D