Goku wasn't even sure that he could beat Fat Buu. But he was sure that Gotenks would at least give Fat Buu a challange. This is SSJ Gotenks, pre-RoSaT he's thinking of here.And that all goes together to show how Goku was clearly exaggerating in order to help convince Piccolo that fusion is the option they should just go with. Since he never did tell Buu that, that's clearly one other thing he just said to keep their hopes up, too. He told Piccolo the opposite of what was true, that he didn't think he could beat Buu but fusion probably could. The truth was he could but fusion was quite a gamble as Goku said himself(it's quite clear he didn't think, "Yep! I'm absolutely positively sure a fighter way, way stronger than Buu who Buu doesn't stand any chance at all against will show up for sure!" ) He must've just not wanted to risk being too honest and going, "I could've beaten Buu if I wanted, but I think they should use fusion, even though I can't guarantee it'll be enough." And get a, "...Get your ass back down there!"
Also, note that SSJ Gotenks wasn't massacred by Super Buu when they fought and Goku said that neither he or Vegeta would stand a chance against him.
Finally, consider the power of Super Buu2. If SSJ3 Gotenks wasn't considerably more powerful than SSJ3 Goku then Super Buu2 wouldn't have been that much of an improvement, would he? Gohan would have still trounced him.
