[module.global.frag] p3 says

A declaration D is decl-reachable from a declaration S in the same translation unit if:

• [...]
• there exists a declaration M that is not a namespace-definition for which M is decl-reachable from S and either
• [...]
• D declares a namespace N and M is a member of N, or

The example after this rule has a case that:

template<typename> struct S;

template<typename, int> struct S2;

constexpr int g(int);

template<typename T, int N>
S<S2<T, g(N)>> f();             // S, S2, g, and ​::​ are decl-reachable from f

The declaration M(e.g. S, S2, g) is decl-reachable from S(e.g. f) and is a member of ::, however, ​::​ the global namespace does not have a declaration per [basic.namespace.general] p3

There is a global namespace with no declaration; see [basic.scope.namespace].

The comment is just a bit misleading. It appears to me that the emphasized wording only applies to the namespace that has a namespace-definition.