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.

An April Fools gag swaps the Kuroko's Basketball cast for its bird mascots in a mock trailer for the Last Game movie.