While it is correct that Fusion definitely wouldn't be strong enough, nobody knew Uub was still around until he briefly re-appeared on Planet Tuffle, where he was promptly eaten. What we do have, for sure, is that
SSJ3 Gotenks is beneath SSJ4 Goku. So far beneath Goku that they wouldn't stand "a silver of a chance" against Golden Oozaru Baby. For reference, Goten's body was considered an
upgrade from Baby's ordinary state in Episode 23-26. Trunks was also considered
superior to infant Baby on Plant Pital. And Infant Baby had the power of Meta Rildo, who was roughly
Episode 18 SSJ Goku level.
Taking into account that Goten and Trunks are roughly considered equals, this implies SSJ3 Gotenks, who would be OVER 100,000,000x
*1 stronger than Episode 18 Base Goku, is stomped by Baby. Also consider that by the fight against Baby on Earth, Goku is
only like, at max, 2500
*2 times stronger than his Episode 18 Self.
Divide those numbers and you get the hypothetical Gotenks still roughly over 40K stronger than E29 Goku. Which fits the fact they would've had no chance against Golden Oozaru Baby, who was stomping SSJ3 E29 Goku as just SSJ, and
stacked his own SSJ3 multiplier on top, and then stacked Golden Oozaru, which took Goku to SB2 Level (meaning another 3200x jump). With all of that accounted for, Baby was still nearly a SSJ3 jump ahead, (512x stronger.)
*1: The amp given by Base Gotenks in the Buu Saga relative to his Fusees, combined with multipliers and the scaling Goten and Trunks have. So 5000 (Fusion Base for Gotenks) x 400 (SSJ3), or 2 million, times 50x because that's SSJ E18 Goku, not BASE E18 Goku, making 100 million.
*2: E29 Base Goku > E28-29 SSJ Baby Gohan (by 50x) > E26 Suppressed Base Gohan >~ E26 SSJ Baby Goten ~ E23 Base Goku > E23-27 Base Goten/Base Trunks >~ E25-26 Baby >~ SSJ Goku/Meta Rildo in E18 (by 50x) > Base E18 Goku. In other words, two 50x jumps, becoming stronger than those superior to his SSJ self against Rildo, (Goten and Trunks) and stronger than someone 50x that (SSJ Gohan). It should be noted that Baby modifies powers and amplifies them when he takes you as a host, but there's no way to quantify that outside of
Baby Vegeta himself, because we can actually compare him to Vegeta, so everyone's likely somewhat higher in the chain. Especially with wiggle room like the fact Gohan was holding back significantly against SSJ Baby Goten.
Point being, we have a pretty solid Fusion chain/piece of evidence, and it doesn't rely on Uub, because why would it rely on Uub? Nobody knew he was there. Additionally, Uub in E1 and Uub when he fights Baby, even without Majuub, are two TOTALLY DIFFERENT levels of power. Uub is equal to E1 Goku, but Goku gets substantially stronger throughout the series. Functionally speaking, even if they WERE talking about Uub, the Uub in their minds would be so far beneath the Saiya Power Scaling Chain created by the Baby Saga you'd net significantly lower than this.