Nobody stuffed any ballot boxes. Simon and his people were ignorant and paranoid. That's the meaning of that quote.
But don't take my word for it. Ask the author of the article.
It means exactly what it means... every single election cycle in CA, the Republican jumps into an immediate lead based on the absentee ballots, and everyone on the FR goes nuts thinking the Republicans are winning. It is really sad. Then the lead grows as GOP counties across the state reports. Once the absentees and GOP counties have reported in and things look really good, then the Rat counties dump all of their votes, take the lead, and win.
Always helpful to know how many votes you'll need in advance.