The Goku vs Buutenks fight was purely anime filler. It never happened in the manga. As soon as Buu charges Goku, Gotenks' fusing wears off.
He powers down because Buu was weak enough for Gohan to handle, which is why he says "Gohan can beat you now". Goku never intended to fight because he knew he'd get trashed. This later confirmed when Goku tells Vegeta they stood no chance against him and that they would die.
Moreover, we see him cowering with hands above his head as Buu charged him.
Yeah, the fight was purely filler, but so are the films. What's the point of bringing up the manga? Two of the enemies here are purely from the films, one of which flattened Gohan in his most powerful form, and knocked SSJ3 Gotenks out of his fusion.
And I'm not arguing anything about Goku vs Buutenks. We all know the result of that fight.