I scanned the websites of CA's 3 statewide homeschooling organizations and Home School Legal Defense Association yesterday and did not see any comment about a surge in interest in homeschooling. All have general helpful info, but those webpages have been on those sites in some form for years or decades.
I think you're right, that it won't be known how many parents decide to continue homeschooling until PSs re-open (January, 2021 at earliest, I also expect). I do get the impression that some union and ed school people are nervous and making clumsy preemptive intimidation noises.