I get the impression that 'Dardic' and 'Nuristani', as well as the older,
now politically-incorrect 'Kafiric' are approximate synonyms.
Nuristani seems to be the preferred modern term. This language group is
usually said to stand with Iranian and Indic as an independent, first-level
division of Indo-Iranian. Others suggest it is a highly
conservative, isolated Indic dialect preserving some astounding
archaisms.
The online Britannica article 'Dardic' is none to clear on this. It's as if
the Dardic group is broken into two distinct units, one of which is Nuristani,
with the remainder treated as Indic -- to include Kashmiri.
My geographic knowledge of this part of the world is still murky, but it
seems the Dardic group is way up in the mountains, tucked into isolated valleys,
just the kind of place where linguistic conservativeness occurs.
Mark.