> and so I read the YAML specification

but there isn't a single YAML spec, there are at least 2 in common use: yaml 1.1, and 1.2, which have discrete specs and feature-sets. re: anchor stuff specifically, 1.1 supports merge keys whereas 1.2 explicitly does not, so that's one thing

and github actions does not actually specify which yaml spec/version it uses when parsing workflow yaml files

it's unfortunately just not the case that "YAML means something" that is well-defined in the sense that you mean here