Where? I never heard this statement anywhere, not like Mr. Buu is a thing to compare Ultimate Gohan to since he styled on Super Buu who was already a mixture of the Evil Buu that soundly beat Fat Buu.
Considering he says nothing else on why and how and we see nothing indicate as such, and said statement sounds more like he was just offhandedly saying "Goku is Number 1!" to hype up Goku for fans, I doubt the statement means anything on it's own.
He beat the shit out of Super Buu easily and only started to lose against Buutanks.
Your own statement says he tried to make Gohan the MC which considering how much hype was going into his training and eventual transformation into Ultimate Gohan makes sense which is why him saying he is not "MC material" sounds like him making up an excuse to cater to fan demand. Even if not it is a stupid ass decision since Goku at that point did not fit as a MC anyway as he had grown up to the point he had no real arc anymore and fit more as a mentor like he did in the Cell Saga and just having him take the spotlight was plain boring and did not fit with his goals in that arc trying to train the next generation to take his place.