The multipliers are canon. The Daizenshuu, which are canon, state that SS2 is twice as strong as SS1, and SS3 is four times stronger than SS2. Since SS1 is 50x, SS2 and SS3 are 100x and 400x base.
Also, he didn't say Buu was exactly 1000x stronger than Frieza, he said Buu defeated 4 Supreme Kais who were 1000x stronger than Frieza. And Frieza was 120,000,000 so 1000x is 120,000,000,000.
Your post also seems to be implying that Goku's base strength never increased in between the Frieza and Buu fights, unless I'm misreading it. It wouldn't be a stretch to believe that Goku and the other Saiyans were roughly as strong as a single Supreme Kai in their base form, considering the S Kai thought they should work together to defeat Babidi's minions, meaning he didn't consider himself strong enough to defeat them on his own. Considering Vegeta beat Pui Pui in base and Goku beat Yakon just by transforming, we can tell that the Saiyans in their weaker forms are already stronger than 1000x Frieza.
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The first time I saw him, I thought "rabbit", but I mostly see him as a jackal, similar to ones like Anubis from Egyptian mythology.