The HTML+RDFa 1.1 Doctype (which will be valid for HTML4 & 5) is still under development:
As such The W3 validator doesn't recognise it, even though your document might well validate against the standard itself.
So to answer your question, yes, Drupal (or what's more likely, the theme that you're using) is conforming to W3C standards, it's just that the spec hasn't been ratified yet.
You might see advice out there telling you not to use that
<!DOCTYPE html> should be used instead. That advice is incorrect, per the specification