kreativzirkel's post in onComplete and .totalDuration() ignore an added timeline via .add() was marked as the answer
Carl has answered the question;
But for better understanding for others that came here searching.
// wrong... .add(vorhang, "-=.5"); // only triggers the animation // right! .add(vorhang(), "-=.5"); // returns the tl object so GSAP can calculate timings correctly