So, in X2. CASTLE AMBER Tom Moldvay includes a brief note on the relationship between this adventure and Clark Ashton Smith's Averoigne stories that, while true, has proved slightly misleading:
Note: The Amber family is not one of Clark Ashton Smith's
creations and does not appear in any Averoigne stories. Their
origin has been traced to Averoigne to aid the continuity of the
module and to ease transition in and out of Averoigne in the course
of the adventure
[p.3, last full paragraph in column one]
It's true that the Amberville line is Moldvay's addition, but the name is one of C.A.S.'s devising, bestowed by him on Francis Amberville, a painter who is the hapless protagonist of "Genius Loci", a modern day story published in 1933 in WEIRD TALES.*
So the name is authentically Smith's, while the family is Moldvay's.
*and later title story for Smith's 1948 Arkham House collection GENIUS LOCI and other tales.