Somewhat more interesting, although it didn't fit on the explanation panel: the choice of ending song was explained as a coincidence by the production team, since they chose the song way back in ~2021 for its nostalgic factor and because of some thematic resemblance between the show Abarenbou Shogun and Akuyaku Reijou Tensei Ojisan. Then, around the time the anime aired (Winter 2025), a remake of Abarenbou Shogun, starring the same actor, Matsudaira Ken (the artist behind Matsuken Samba II), was also airing.
Combine that with the actually great and catchy remake, the nostalgia factor, and the song's status as a cultural phenomenon in Japan (since the majority of Japanese people know it), along with the surprise factor (because the ED was never announced), and it became quite a buzz in Japan.