This is one of the most debated things in Brasília. The current understanding is that the design is neither an airplane nor a butterfly, but simply a cross adapted to the topography of the region [1]. The confusion comes from the words "Plano Piloto" (literally, "pilot plan" or "pilot design"). The butterfly metaphor, apparently, was a lyrical comment by Lúcio Costa, not a literal source of inspiration. Anyway, the intersections do resemble butterfly wings [2].

[1] https://g1.globo.com/df/distrito-federal/noticia/2019/06/04/... (Portuguese only)

[2] https://www.dronebrasilia.com/assets/img/portfolio/agosto201...