Nobody watching the actual Last Game trailer expected the bird mascots to hijack it. That's the entire joke, delivered with such commitment that it stops being a gag and becomes its own thing. The energy is pure April Fools' chaos, but it lands because it respects what it's parodying enough to make the bit feel earned rather than lazy. This is fanservice for people who actually know Kuroko's Basketball deep enough to care what the mascots were doing in the background.
2017 April Fools' Day event where the Kuroko's Basketball bird mascots fill in for the characters in a trailer for the Last Game movie.