What are the circumstances of Traxigor’s transformation and do they explain how he retained his magical abilities as an otter?

Traxigor is an Archmage with a rather unique morphology, who is encountered during Baldur’s Gate: Descent into Avernus:

Traxigor was polymorphed into an otter years ago, and decided he preferred the new form to his original one (that of a wizened old man). His otter form was made permanent by a wish spell. He uses the archmage stat block, but is a Tiny beast with a Strength of 3.

What exactly happened here? Are the circumstances of this transformation recorded in any official source material (from any edition)?

This answer establishes that (in D&D 5e) the spell polymorph would not allow a spellcaster to retain their spellcasting abilities while polymorphed into a beast, and this answer establishes the same for true polymorph. On the surface, this seems like an inconsistency (which wouldn’t be surprising), but do the circumstances of Traxigor’s transformation into an otter resolve this inconsistency somehow?