The Sprogs are real but Billy Bob isn’t an alien.
Billy Bob isn’t an alien but the twins are.
Twins are the real aliens not Billy Bob but they didn’t know their true origin. The real aliens are the members of the band Bis.
I mean, think of how popular the Powerpuff Girls was! That's got to be aliens! Perhaps they only don't use the powers that Billy Bob does because it's not normal to do these things as an adult as opposed to a baby. They definitely use them in private, however. This show is the reason the band Bis broke up.
They considered it their Crowning Moment of Awesome and knew that it was best to hang up the boots right then. Or, maybe they realized the show was on to something, given how sound the aforementioned WMG's logic is... Think about it.