You've misunderstood, from your own article:

“California’s forests naturally adapted to low-intensity fire, nature’s preferred management tool, but Gold Rush-era clearcutting followed by a wholesale policy of fire suppression resulted in the overly dense, ailing forests that dominate the landscape today.”

It doesn't say 20 million acres need to be removed, it says that's the area that needs burned. Low intensity fire clears fuels without killing mature trees, this is especially true for long lived redwood species that expect fire to clear out smaller vegetation below them. It's not about removal or clear cutting.