That's the same level I had when I got very ill, it's near the bottom half of the insufficiency range, you should definitely consider supplementing and monitoring it.
Per your question, the latter, the theory is autoimmunity is exacerbated in a certain genetic makeup, and to compensate for it by megadosing.
You mean for compensating for the low vit D levels due to constant inflammation seen in autoimmunity? or to decrease autoimmunity? I have autoimmune diseases and my vit D goes down to about 7 ng/mL without supplementation, so I'm curious.