Yes, that is what my tests showed. Missing doctype does get complained about on load of epub but missing xml header but with doctype will load without warning.
Mend will properly fix this.
I have no idea why epubcheck does not detect this issue, and Sigil pre-1.0 automatically fixed this issue. The auto-fix got broken when we let people just load things with no changes in Sigil 1.0 - 1.3. The epub will still work correctly if no named entities are used (this is how calibre does it I am told). But then people stopped adding the doctypes but then started using named entities for copyright, nbsp, smart quotes, smart apostophes, etc and that broke things.
So for Sigil 1.4, we now explicitly check for the doctype and warn the user that something is broken if it is missing.
Here is the spec cited by epubcheck that says a DOCTYPE must be used.
https://www.w3.org/TR/xhtml11/confor...html#s_conform