Not so easy. There would initially be chaos, but many people would be able to flee. The SF peninsula is blocked by water on 3 sides, but not so in the greater Bay Area. And even with major choke points on roads on the peninsula, many out of a couple million can flee on alternative roads, or make their way through the wilderness trails along the peninsula (only a small sliver is populated). The forested areas have lots of game and fresh water lakes and streams, so smart people won't starve (smart people being the minority of conservatives here and we are armed). It is even easier to flee on the other side of the bay.
I've lived and hiked in all parts of the Bay Area all my life. I have done detailed habitat restoration in this region for the last twenty years. I know this country intimately. You don't know what the hell you are talking about.
San Francisco and the peninsula are obviously landlocked, but for the most part they are soon contained by chaparral so dense you can't reasonably force your body through it. Burn it and you'd have a conflagration with easy pickings thereafter. There's no summer water on most of that land and almost nothing to eat because it is so heavily overstocked with decadent oak/madrone woodland. To take the crowd down, all it would take is a drip torch.
My wife and I spent months exploring escape routes out of the South Bay. The Mid-Peninsula Open Space District had allowed EVERY trail on the map that went outward to the South and West (toward our home) to become overgrown. They were all gone. To the East of the South Bay (south of Sunol) is the Diablo Range. Good luck there. Not only is there chaparral on many of the eastern slopes there's little to no water in the summer except in reservoirs where people would be easily rounded up, worse if they poison them. To the East of the Bay Area proper, after crossing the hills, again with dense chaparral is the San Ramon Valley, where they'd run into similar chaos. To the east of that it is again dry as a bone. To the North of the East Bay is the River. No crossing that.
I don't buy your contention.