A lighthouse is a structure that emits a light that can be seen from a long distance away, so the simile is comparing the fact that he can sense her mind easily despite the distance as something bright and welcoming not dark or foreboding
Consider the statement "He could pick up her mind from miles away; it was always dark and foreboding.". Based on this "He could pick up her mind from miles away; it was like a lighthouse beacon to him." a true statement? Yes, no, or I don't know?