These closures are sometimes misleading. Right now, Mount Diablo State Park is closed...to vehicles. It's actually open if you're on foot or bike:

UPDATE (May 1, 2020): This park is temporarily closed to vehicular access, meaning there are no parking facilities and parking on roadways is prohibited to protect public health from the COVID-19 pandemic. Although this park is open to local residents, they must abide by the following guidelines:

• Stay Local: Walk or bike into the park. Do not drive to the park.
• Stay Active: Keep walking, jogging, hiking and biking. Watch for one-way trails.
• Stay Safer at 6 Feet: Maintain a physical distance of 6 feet or more. Gatherings, picnics and parties are not allowed. Visitors are being asked to leave if there are too many people at the park or on trails to allow for the required physical distance.
• Stay Clean: Be prepared. Bring soap/sanitizer and pack out all trash.

Statewide, many parks and beaches are temporarily closed or have very limited access to ensure Californians are abiding and practicing physical distancing. The goals are to make sure people are safe and to mitigate the spread of COVID-19 as much as possible. To view the list of closures and what they mean to the public, please visit www.parks.ca.gov/FlattenTheCurve.

https://www.parks.ca.gov/?page_id=517

Adding to the confusion is that parks and beaches in the state fall under federal, state, regional, county and city jurisdictions. Bottom line is if you're thinking of visiting a park or beach, go to its website to see if it's completely closed (as in stay away), or just closed to vehicles. Also, closing parking lots can, as a practical matter, mean stay away because there is nowhere to park close to where you want to go. That's the situation now in Mt. Tamalpias State Park. I rode my bike to the summit a couple of weeks ago, and I didn't see anywhere to park for hikers.