FontAwesome have at some point made new icon names to avoid the confusion:
https://fontawesome.com/icons?d=gallery&q=sort
It doesn't mention ascending/descending anymore.
I think it's mostly to do with the two different parts of the icon. The arrow and the "ordered" objects. Traditionally (e.g. in Windows Explorer), the any list is assumed to be sorted by its type's natural ordering such that the "biggest" element comes on top. Then the arrows are basically: do you traverse from top to bottom (hence: descending) or from bottom up (hence: ascending). This is the common mathematical intuition.
The confusion (shared by FA) comes from assuming a physical order for a list, with the default being smaller-elements-first. That's why "ascending" icon was pointing down, as it was traversing the list in its current order (which is smaller-first).
TL;DR: FontAwesome's original icons were thinking of lists as physical, with the natural order being _both_ (physical: top to bottom) and (logical: small to large). While most other software assume only (logical: small to large) with the physical order being implied by the UI element (e.g. what do you do with side-scrolling lists?).