And those same Goku & Vegeta also agreed that not even Gohan's energy given to its very limits could make the job, then we have two contradictory statements, but statements can't override facts.
For your premise to be true we have to go by "Goku has to be right", not because he was proven to be right.
The fight goes down like this: Kid Buu regenerates from Goku's Kamehameha, then he gives an elbow that doesn't do much, Kid Buu use his leg to kick him, tries to fire a blast but Goku avoid it using IT, then proceeds to kick him, regain posture and proceeds to punch him in the face making him bleed, then they traded blows, managed to launch a kick, proceeds to bit him, which Goku does too, use his chocolate beam, Kid Buu's copy his Kamehameha while laughing, Goku does the same, proceeds to spit bleed and Kid Buu regenerates, then Kid Buu proceeds to dance.
Doesn't give the impression like he is treating him as a threat at all, while Goku was willing to go all-out since the very beginning. Since Vegeta is too weak to fight any form of Majin Buu it makes sense for Goku to be the only one capable of fighting Kid Buu.
Goku thought that the Genkidama would kill Vegeta, but ended failing on killing a heavily injured Vegeta at the end with half of it, Goku believed that the Genkidama could kill Freeza and that Freeza was telling him lies when he says that he is using just half of his power, just to realize he was telling him the truth and end failing, thing is, that your premise is inconclusive if we go by the merit it holds.