At least for gondolin and microsandbox, you bind a specific secret placeholder to the target host. i.e. your GH token is only replaced/injected for calls to api.github.com, not other hosts. And you can set up both with deny-by-default
The main reason a "proxy-managed" env var is set is because most CLI tools assume if the env var is set, auth is set. If the env var is unset, it will assume auth needs to occur. Fortunately, most don't do a pattern matching on what the value actually is.