[The inhabitants of Salmalidvipa worship the demigod of the moon in the following words.] By his own rays, the moon-god has divided the month into two fortnights, known as sukla and krsna, for the distribution of food grains to the pitas and the demigods. The demigod of the moon is he who divides time, and he is the king of all the residents of the universe. We therefore pray that he may remain our king and guide, and we offer him our respectful obeisances.
You misinterpreted the verse. Verse says with Moon light, Soma distributes food to manes and Gods. It also says Moon God divides the month into two fortnights. It doesn't say Moon is self illuminating. SB 5.20.12 says
स्वगोभि: पित्रुदेवेभ्यो विभजन् कृष्णशुक्लयो: |
प्रजानां सर्वासां रजान्ध: सोमो न आस्तवीति ||
svagobhi: pitrudevebhyo vibhajan kṛṣṇaśuklayo: |
prajānāṃ sarvāsāṃ rajāndha: somo na āstavīti ||
"May the Moon-god who, dividing (the month into) dark and bright fortnights, by his rays distributes food to the manes, gods (respectively in the dark and bright fortnight) and to all created beings, be our ruler.
Translators of verse is Dr. GV Tagare and J. L. Shastri and commentators say their translation and commentary is close to oldest and reputed commentary of Sridhara Swami. Commentators give annotations to the verse as follows:
(i) VR. emphasises that but for the division into dark and bright fortnights by the Moon-god, there would have been no food offerings to the manes and gods'. May the Moon god bring happiness (amrtam = sükham) to us.
(ii) VJ : May the inner controller of the Moon-god whose rays are full of nectar, distribute Soma to manes and gods and food to us all in both dark and bright fortnights. May he bring happiness (somam = sukham) to us'.
(iii) BP. attributes the division in dark and bright fortnight' to the Moon-god. By his rays he distribute: food to the manes (in the dark fortnight) and to the gods (in the bright fortnight). May that Moon god be our king and bring us happiness
So, in no way this verse says Moon is self illuminating.
They pray: ‘Soma bestows by his rays food on Pitris during the dark fortnight and on the Devas in the bright fortnight. May that Soma, the Raja, be propitious to us his subjects! - Bhagavad 5.20.12
This translation is done by Swami Tapasyananda and is based on Sridhara's interpretation. The translation provided in the question is the translation done by Srila Prabhupada, the founder Acharya of ISKCON, following the Gaudiya Vaishnava tradition.
The original verse and word-by-word translation are as follows (taken from Prabhupada's translation)
prajānāṁ sarvāsāṁ rājā-
ndhaḥ somo na āstv iti
Here in this context, the important word is "sva-gobhiḥ", which Prabhupada translated as "by expansion of his own illuminating rays". "Sva" means own.
Thus one can see two great Acharyas of Vaishnava lineages have said "Moon rays" or "Moon's own rays".