You could interpret that differently if you want. Gohan can think he would be vastly superior to Buu, but is worried about bringing out his power, not that he couldn't beat Buu.
He said he could
manage something with true power, I don't see that implying he would end up being way higher than initial fat Boo.
Also, Gohan is not cocky like Vegeta is, we didn't see Goku laugh off Buu's power, yet we know he's just as strong as Vegeta.
Goku didn't despise fat Boo because he noted that was something strange about his chi, probably thinking he could still get way stronger:
Chapter: 461 (DBZ 267), P4.6, P5.4-6
Goku: “It’s a ki…! A huge ki has appeared…! Majin Boo has finally come out…”
Vegeta: “Fuffuffuh…Majin Boo, huh?...Here I was wondering what kind of amazing guy he would be, and he’s got this kind of battle power number?...I thought so…Kakarot, you and I have now become too strong…By a wide margin! By his nature, Kaioshin is supposed to be someone tremendous, but have you ever once thought that he was incredible? Quite the opposite, it's Kaioshin who’s been bewildered…Majin Boo is fearsome from Kaioshin’s perspective, but from ours he’s not so much…”
Goku: “N-no…That’s not it…
There’s something abnormal about this ki...”
That could have something to do with Goku later stating Boo's chi is like a lie, I don't know:
Chapter: 476 (DBZ 282), P10.5-8
Piccolo: “Goku…There’s something I want to ask you while I’ve got the chance…[ ] …That Super Saiyan 3 thing earlier…if you had gone all-out, wouldn’t you have been able to defeat Majin Boo?...How about it, am I wrong?”
Goku: “Nah, I don’t know…
When it comes to Majin Boo’s strength, it’s like a lie…I think that I probably couldn’t have won…”