That's correct with the modern interpretation, but the document does say "legitime", doesn't it? Were Albert's younger siblings the first time "strictly legal" was considered?
King Albert II's half sisters are not in the line of succession because the constitutional amendment that introduced cognatic succession only applies to Albert II's descendants. It is a similar situation to that of King Carl Gustaf's sisters in Sweden. It has nothing to do with legitimacy or lack thereof.
In fact, I suspect legitimacy was not a factor either in the court's ruling regarding Delphine's title. The court simply applied the royal decree of 2015, whose legacy clause says that children and grandchildren of Albert II are princes/princesses of Belgium and carry the style of Royal Highness prefixed to their given names. The royal decree does not mention legitimacy or "children born of a marriage" as is the case for example of the Dutch law on membership of the Royal House.
In short, Delphine got the title because the wording of King Philippe's own royal decree of 2015 (and Baudouin's previous royal decree of 1991) was sloppy. And I suspect the King won't dispute the ruling. As long as Delphine and her children are not included in the line of succession, or given an official role, a royal residence and a state stipend, being HRH Prince/Princess is of little consequence.
EDIT: BTW, the Royal House is not to be blamed for the poor wording of the royal decrees since they are drafted by the government. The responsible ministers who countersigned the royal decree are the ones who should accept the blame.
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