The Nations League positions have been used for the play offs for Euro 2020 and now Euro 24. For the last World Cup the two highest ranked Nations League group winners who didn't either qualify automatically or finish 2nd in their World Cup qualifying group were put in the play offs as well...
Luxembourg may have finished 3rd in their group, but qualification for the playoffs was based on Nations League positioning not where you finished in the Euro groups. In Luxembourg's group Iceland and Bosnia & Herzegovina who finished 4th and 5th have also qualified for the play offs.