If these guidelines are not approved by the committee, why are they under `github.com/isocpp`?

Because isocpp is the Standard C++ Foundation; the committee's repositories are under github.com/cplusplus. Some neutral organization has to own the copyright and license to make it clear this is not being dominated by any one person or vendor. The natural entity is the Foundation, which exists to promote the use and up-to-date understanding of modern Standard C++ and the work of the committee. This follows the same pattern that isocpp.org did for the C++ FAQ, which was initially the work of Bjarne Stroustrup, Marshall Cline, and Herb Sutter and contributed to the open project in the same way.