I would note that Princess Beatrix & the king/queen emeriti of Spain are listed as attendees - none of them are current heads of state, so clearly there's some flexibility.
Also among the non-reigning more than two invites went out to Greece and Baden. And Denmark’s situation was unique anyway.
To what kind of family occasion does one invite a married person but not their spouse? Only a robot-like bureaucrat totally incapable of nuanced thinking would do that.
405