I don't think that's quite true (Where's Tommy when you need him :))
<!doctype this is junk >
It's the doctype element that will trigger standards mode. (Or perhaps to be more correct an unrecognised doctype.)
On its own it is an unrecognised doctype and standards mode is assumed. It's mentioned in Eric Meyers chart from many many years ago.
It's only when combinations of label and url are added that you get the various quirks modes, nearly quirks and standards mode being defined.
Making a typo in these will not always default to quirks mode but ony when the typo nullifies one part and then the remaining part matches the requirements for quirks mode.
This junk will still render in standards mode in all versions of IE.
<!DOCTYPE HTML PUBLIC [B]what is this for[/B] "-//W3C[B]xxx[/B]/DTD HTML 4.01//EN" "[B]Nonsense here [/B]http://www.w3.org/T[B]xxxR[/B]/html4/strict.dtd">
But this won't.
<!DOCTYPE HTML PUBLIC what is this for "-//W3Cxxx/DTD [B]rubbish [/B]HTML 4.01//EN" "Nonsense here http://www.w3.org/TxxxR/html4/strict.dtd">
Obviously in the second example I broke the pattern searched for.
So we can assume that browsers first assume standards mode for any page with a doctype element and then try and match wsections of whatever else is in the doctype against all the known options available.
CSS Enhancements in Internet Explorer 6
Eric Meyer on CSS: Picking a Rendering Mode