I've liked this theory since I first heard it. One thing I might add is that if Big Mom did free him, it might not have been for altruistic reasons, but instead just to have another rare species on whatever Tottoland was at that point.
Now that we know more about the Numbers, what if only Kaido managed to escape Punk Hazard, freed by Big Mom, and left behind his guinea pigs mates, the Numbers? They got experimented on more than him and ended up being much bigger than him. When he got more powerful, after Rocks was defeated, he broke a deal with the WG and bought the Numbers' freedom back. That could also explain why they all drink that much (Kaido too), maybe it's a way to keep at bay some sort of collateral effect of the experimentation, like the kids in PH and that drug (which was also the catalyst for the growth iirc).