These examples sound like XS code exploiting undocumented or internal features, without being widely used.
For a long time, there was no strong line between "documented" and "internal only features" in XS. Furthermore, there's still a persistent belief that CPAN modules should support dozens of releases of multiple stable major versions of Perl going back multiple years.
XS-as-extension language being the same as XS-as-implementation-language is a big blocker to major internal rewrites. If a solution to that had begun in 2000/2001 (instead of wasting a decade and a half producing Advanced Perl Substitute), we might be seeing the benefits of it now.