(SPOILER ALERT)
The true ending is when you use the warp doors to go to the extradimensional space with the warps to the green speedrun things, and get good enough times to unlock the purple powerup thingy. Then, when you go to games end at the top of the tower, you will get that cutscene. The standard ending is when you don't get the ??? powerup. You get a different cutscene at the end, basically.