The problem with all of this is NOTHING even remotely says this is true.
"Goku said that Gotenks will be stronger than him"
Again, he did not. The scene shows Buu asking Goku is this new fighter will be strong (not stronger, not as strong, just strong) Goku ignores the question.
Goku tells Piccolo a few lies, including that he doesn't even believe he could stop Buu, and follows up with it's a GAMBLE to rely on fusion and the kids.
Krillin, Roshi and everyone else are surprised at Gotenks Ki in base
Piccolo is impressed at Gotenks power as a SSJ - in neither scenario did they compare his power to Goku OR Buu.
Later Krillin and Piccolo have a discussion about Buu and Gotenks, with Krillin stating <b>"Goku said</b> fusion is the strongest" as it's not something he observed himself personally. Piccolo expresses his own doubts as well, stating "That would be nice, <b>BUT..."</b>
Even in ROSAT, when Gotenks fails to beat Buu, Piccolo comments that Gotenks did better than he expected, meaning that him even managing to fight near Buu's level wasn't something he expected him to be able to do.
Gotenks is substantially weaker than Goku, and according to the guidebook (daizenshuu 7) he doesn't even surpass Vegeta until SSJ3