Mahābhārata (Mahabharata) at one place refers to Aṃśumāna (Angshumana) as the lord of the entire world of herbs or creepers— vīrudhāmaṃśumantañca (virudhamangshumantanchak). Hence it appears that this Aṃśumāna is definitely the Moon or Soma, who is widely known as the lord of medicinal herbs— puṣñāmi coṣadhī sarbāḥ somo bhūtvā rasātmakaḥ.
Apart from this, the description of Soma chronicled in the Vedic ages mentions aṃśu (angshu) or radiance as one of the many characteristics of its medicinal properties.
[Ex. R. Gordon Wasson; Soma, the Divine Mushroom of Immortality]