@owashii@snouts.online okay, apparently, in the original Cell timeline, the one we never see, that Cell came from, which is utterly unaltered, Goku beat up and killed Mecha Freiza and King Cold, which is where the DNA was harvested from. It's possible Goku didn't go Super Saiyan there because he didn't need to, and officially Gero's super computer was doing all the harvesting work, so didn't get newer Saiyan DNA.

However, more likely, that's a bunch of handwavium powered justification for "fuck forgot that"