It is not clear who is writing this account. It is in the third person, so it was not Ammon. It was based on Ammon’s record, which Alma had. The two options are Alma and Mormon. It is probable that this is Alma’s account, rather than Mormon’s.
This verse might be the clue that shows us that it was Alma. Alma had a similar experience with the spirit; this description of the war between darkness and light in Lamoni’s mind has the feel of a lived experience. It feels like Alma infusing Lamoni’s experience with the similarities to his own experience.
Note the “dark veil of unbelief.” For Lamoni, it was cultural and inherited. For Alma, it had been an intentional rejection of belief. Both led to a type of darkness. That darkness came into contact with “the light which did light up his mind.” Perhaps Lamoni explained this to Ammon, who wrote it, and Alma simply referenced it, but it still feels more like Alma imputing his own dramatic experience to what was also a dramatic experience for Lamoni. The imagery of light infusing his soul with joy, that it was the “light of everlasting life,” which “lit up his soul,” has the feel of being written by one who really knew what Lamoni experienced.