Charles Shultz lived in the Bay Area, so I assume he was a Giants fan.
He might have been both.
I also recall the Alvin Dark references but also a rare current-events reference when an anguished Charlie Brown wails ‘Why couldn’t McCovey hit the ball two feet higher?’
In another episode, Charlie Brown blows a spelling bee contest when asked to spell the word “maze.” Without thinking, he spells it M-A-Y-S.