Ammon knows much of this situation, not only by the Spirit but because he had watched over the unconscious Alma2 (Mosiah 27). In fact, Ammon was preaching to Lamoni as a direct result of his experience with Alma. Ammon knew that Lamoni was alive and also knew what was happening to him because it had also happened to Alma.
Variant: The original manuscript and 1830 edition both have “that light of everlasting light was lit up in his soul.” After careful analysis, Skousen suggests that this was an error created by the large number of occurrences of light in this passage. The 1981 edition has a correct life, which appears to be the correct reading of the text, supported by other internal evidence.