An ADU has sleeping, bathing and cooking. There is another structure name for a space that is only sleeping and bathing. In WA we have ALQ or Accessory Living Quarters. Basically a place to put your annoying high school child. The NEC will probably have to catch up with terminology to make the requirement more clear.
Does it? 210.12(A) lists certain room types, then follows those with "or similar rooms or areas".
If anything they just need to change 210.12(A) title from "Dwelling units" to somehow include similar areas located on dwelling properties.
I could also see somehow incorporating similar areas on non dwelling places like camps, recreation areas etc that have bunkhouses or similar sleeping rooms whether within a single structure or as individual structures. Those spaces might be a little questionable as is worded now whether to consider them dormitory areas or guest rooms per 210.12 (B) and (C), but is still similar in use/function as areas that currently require AFCI protection.
I say this even though I am for the most part anti AFCI, can you believe that?