"Cefni" was "Kefenny" in 1766 (*p.16) and "Kyffyn" is recorded as a local name at least once (*p.6). The evolution of the name of the valley in the manuscripts (Note) draws comparison with names like "The Chevin" in West Yorkshire and the "Cévennes" in France (which probably gave their name to the local Chenin grapes, who's name matches one of the manuscript variations). As unlikely as it seems for a river, the name is probably from the same derivation as modern Welsh Cefnen ["Ridge","Hillock";"Bank"]†; one can only presume because the stream runs through such a ridge-strewn area or below the prominent ridge now followed by the B4421 (Cefn Mawr?). It seems likely that an appropriate translation for the wonder would actually be "walks at night-time above the valley of the ridge(s)".
†Also, possibly Cefn "Back". Though "Cenin" c.876+ CE in Book of Llan Dâv (Note from Wonder 6) seems to suggest a final "n" has been lost, the variations are too great to really draw a distinction, nor is the difference important as Cefn is equally used to describe back-like ridges and hills.