Annex K is optional; it says so itself.
See K.2 paragraph 2:
An implementation that defines
__STDC_LIB_EXT1__
shall conform to the specifications in this annex.
with a footnote:
Implementations that do not define
__STDC_LIB_EXT1__
are not required to conform to these specifications.
And paragraph 3 says:
Subclause K.3 should be read as if it were merged into the parallel structure of named subclauses of clause 7.
which is why it's not necessary to mention it in the library section, clause 7 (or at least the authors of the standard didn't feel it was necessary).
An implementation that defines __STDC_LIB_EXT1__
must define it as 201112L
; both N1570 and the released C11 standard got this wrong, but it was fixed in a Technical Corrigendum.