The Wikipedia page references Günter Wetzel's website. Reading https://www.ballonflucht.de/en/missverstaendnis.html , he's written down more information - apparently they came to a disagreement about the story, and on that page there's more detail, from his point of view: for the first attempt they did everything in Wetzel's house (whose oldest kid was 4), because of concerns the Strelzyks' children (who were 10 and 14) could blurt something out. After some disagreements (Strelzyks told some relatives about the balloon, Wetzel thinks the balloon was too small) Wetzel gave Strelzyks all they've built (he was also worried about getting caught with the stuff, especially since now the relatives have heard about it) and decided to follow the concept of a ultralight airplane.

That's the reason the first attempt was just the Strelzyks...