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Sigil is making the most spec compliant and consistent epub it can. According to the spec, DOCTYPE is required and older versions of Sigil quietly fixed this on load as it had to move things to fit Sigil's standard form. Newer versions of Sigil no longer auto fix the missing DOCTYPE (but Mend will properly fix it) so it warns the user to fix things and offers to auto fix for them.
Those same e-readers will work just with the DOCTYPE. As I said, epubcheck has an open issue to fix this.
BTW, any epub2 that has and uses any named entities (ie like nbsp) in it that is missing the DOCTYPE is technically broken and will not work on most e-readers because epub2's version of the DOCTYPE is where the named entities are included.
That is why this is important to fix.
Calibre is not spec compliant on this issue but does replace all named entities with their numeric or character equivalents, which makes not having a DOCTYPE even on epub2 possible but technically against the rules.
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