It's not really "special pleading" when the stakes are different. Biden didn't need to focus on maximizing votes in CA vs. doing so in GA. If we had a straight popular vote for the presidency, your argument would work. Since the Electoral College is what matters, fraudulent efforts will generally be aimed at a handful of states.
So while fraud can still potentially occur in CA (perhaps with regard to local offices), if we're focusing on the presidential election, CA isn't relevant.
And while it is true that CA did have substantially more votes in 2020 than in 2024, it could certainly be a case of fraud, but again, that wasn't my focus in the discussion. In short, the potential fraud involved is less consequential on the presidential level.
Special pleading in the context of this discussion would be me assuming that fraud can only be committed by Democrats. I haven't made that assumption.