In the manpage for strsep, it states: The strsep() function returns a pointer to the token, or NULL if delim is not found in stringp. It seems that if the delimeter is not found, that it returns a pointer to stringp, not NULL, which would make much more sense for parsing delimited text.
The info page has better (and more correct) info, still pondering what to put in the man pages.
Fixed in man-pages-1.34-2, coming soon to a rawhide near you.