Except this particular beach happens to be a very abnormal one. This issue was resolved years ago, and Khosla won: https://www.eastbaytimes.com/2016/04/27/martins-beach-appeal.... In 2016, the California Court of Appeals held that, because Khosla's property dates from a Mexican land grant, and the state never reserved an easement, there is no public easement on Khosla's property. In 2019, the same court rejected the alternate theory that an easement had been created by the property owner "dedicating the easement to public use." See: https://www.bloomberg.com/news/articles/2019-11-26/billionai...
As you recognize, the "public easement" theory is what applies here. But California lost on that theory. There is no easement here. California has been attempting to keep the beach open using unrelated regulatory powers. (Basically, there is no public easement, but through our powers to regulate what can be built on the beach, we can keep you from building anything to keep the public out.)
If it wasn't for the wildly misleading reporting, everyone would recognize this as overreach and abuse by the State.