Both those mechanisms could be in effect simultaneously.
I feel the second (atrophy or destruction of myelin sheaths), if it occurs, might occur in some individuals and not others due to genetic factors affecting robustness of the sheath.
I can find no research demonstrating clinically significant myelin damage or neurological impairment in humans due to long-term statin use.
Also, (Claude AI):
“Humans have more complex compensatory mechanisms that may mitigate myelin disruption compared to rodent models.”